DISCLAIMER:
This review is entirely for my own sake. You are welcome to read it but it may or may not contain spoilers for the whole game.

I went in with high expectations having heard so many people talk highly about it, and I have to say I was a bit let down by it. Let me try to explain why.

Firstly, I will say for clarity's sake that I did 3 playthroughs being as thorough as posible in all of them looking for all the items and dialogue I could find by backtracking quite often. I started with a pacifist one, then I got a neutral ending by accidentally aborting a genocide run in the last area, and I finished by correctly acquiring the genocide ending. I then looked up online some secrets and FUN value events, though I might have missed a few.

The premise is interesting, the surprise when the path diverts from that of Undertale is well executed and having Flowey helping you throughout the journey makes sense.

The Dark Ruins were fun to play through. The enemy designs and attack patterns start out strong, very varied and great; though the writing, the jokes and the puzzles fall a bit behind. The miniboss is a good tutorial for blue and orange. The Micro Froggit/Golden Pear secret was particularly good and satisfying, and although it's a recurring joke, it never gets as complicated and fun to find again. Dalv felt like a nothing character, and even further in the story he stays being just tangentially related.

Snowdin continues with great enemies and lackluster puzzles. The addition of NPCs makes for more opportunities for jokes and side-quests, which is great. We meet Tom Nook Mo, who is a simple but funny enough recurring character. The mail feels like a good addition, but ends up being heavily underutilized, it might just be an excuse to have fast travel at the end of the game. The Resort is pleasant enough with good foreshadowing. And we finally meet our main companion on the adventure, Martlet. I've read comparisons like "we have Papyrus at home", but I disagree. She has more personality, backstory and takes a lot of actions. She even has a character arc no matter the run you choose to do. This might be a good opportunity, however, to mention that all bossfights boil down to "dodge until they run aout of dialogue" with no puzzle to spare them.

We then take a detour to the Dunes. Basic enemies knock it out of the park, as usual. NPCs take a bit of a backseat until the Mines, where combat is the one that disappears.Moving on... oh, right! El Bailador exists. It's a nice change of pace, but it could have been so much more. Oasis Valley is very mediocre and forgettable except for:
a) The Mew Mew arcade game being good practice and foreshadowing for all final bosses (though it lacks a reward for beating it).
b) Undertale Red making an appearance. It's very minor, but it meant a lot to me so I HAD to mention it.

Next is the Wild West. I just didn't like it. The Feisty Four were good, Starlo is ok, Cebora doesn't exist until later in the pacifist run. But the whole plotline felt both rushed and dragged on. I had more fun seeing what the minor NPCs were up to between each Mission than with the main story during this section. (This is where the joke that made me laugh the hardest occurred. At one point looking through the hospital window results in something along the lines of "The doctor is cowering in the corner while a patient eats an apple.") The fight with the Feisty Four seemed good, but I wish I had died so I had to do it again with more dodging and less tanking so many hits. Their team attacks were fun, would have enjoyed more of them, other combinations and maybe all of them at the same time. The fight with Starlo, however, was very good. The lasso was a good mechanic (although I don't recall him having one before. Missed opportunity to foreshadow it). The conclussion was ok, leading to Ceroba taking the leading role replacing Martlet.

Steamworks is a place. The enemies suddenly feel uninspired. AXIS feels like a Mettaton rehash with no personality. Even the vendor fails at being an entertaining character and it had every chance to be one, being a tsundere vending machine. I can't imagine how boring this area must be in a neutral run without Ceroba dropping lore bombs left and right or Flowey starting to become scared of Clover. Having Cebora give you protection from a hit regularly is fun, but wholly unnecessary. Maybe if the faights were harder it would be more justified. Apparently the path changes in the netral route, but still quite boring. The bossfight shield mechanic is fun both on the ground and on the circle. The only problem is that the RNG of the attacks and the lasers sometimes makes damage unavoidable. Not my biggest gripe since I don't usually care for no-hitting, but it's annoying when it happens multiple times in a row.

Onto the finale. The revelation of Ceroba being untrustworthy took me by surprise. Chujin's house and tapes were nice. The small Hotland and MTT Hotel UG Apartments were well used without reusing every joke from Undertale. Martlet helps us chase Starlo and Ceroba to New Home and she attacks everyone. Her fight has a lot of RNG but it also has many checkpoints so it never really feels impossible, just difficult. Very fun seeing how powerful she is and all her phases. We get flashbacks of her past, Chujin and Kanako. Her motivations are then clear and reasonable. After beating her, choosing to spare or to kill leads to two different endings in which Clover sacrifices themselves in order for mosters to be free when the next human falls. A just ending.

Before moving onto the genocide route let me briefly mention that in the neutral route Flowey betrays you at the very end killing Martlet and absorbing Clover's soul. The Meta Flowey fight is pretty much an Omega Flowey redux, not much better nor worse, but fun nonetheless. He toys with us pretending to be beatable but ends up offering resetting the timeline to find a different ending.

As for genocide, every enemy has a hurt sprite which makes everything sadder. Interestingly the stock of items you can steal is limited, which makes resource management a thing you have to worry about, since there are 4 relatively challenging bossfights.

1) Martlet is no pushover, she doesn't go down easy. You are not high level, so health is low, your items are trash that barely heals and she hits like a truck. You have to learn to dodge them. Fun fight, took a couple of attempts.

2) Ceroba (wait already? Yes, and you are only level 11) is arguably harder/easier than her pacifist counterpart. I don't know if I found it easier or harder since they took about the same amount of attempts, but I can definitely say that this one was WAY more fun. No RNG, just hard patterns you have to memorize and learn how to dodge. She has attacks that freeze you in place, attacks that reduce your max health and a second phase at half health where her attacks become tougher and the walls reduce your max HP without a checkpoint in between. I loved this fight so much I beat it twice and I'm sure that I coul beat it heal-less if I tried hard enough. And I might.

3) AXIS is a surprise because instead of being chased by him, you chase him around to finish him off while he warns the other robots to hide. And instead of you using a shield, you have to attack him until you break his by learning to shoot with your soul and magically getting like 7 levels out of nowhere. Not super tough, but healing items were used.

4) Martlet is no pushover, she doesn't go down easy. This is what I meant, she's back and with a syringe with what I presume is DETERMINATION taken from Alphys' True Lab. She has two phases with a checkpoint in between, thankfully as both of them are hard on their own. The first involves using your new shooting ability and an even newer dash to take down her armor preventing you from attacking. You have to use your healing items wisely because her attacks are chaotic and you have to split your attention on shooting dodging and dashin and it's too many buttons to do it perfectly. The second phase involves using your shots and your attacks to reduce her health to zero, but you can't attack too often because most turns will be spent on "Endure" in order to try to outheal the damage you receive in increments of around 20. It's a good fight, but I probably would have enjoyed it more in auto-shooting mode to be able to focus on actually dodging and dashing instead of spamming, tanking and hoping not to get hit too much.

After that ends you absolutely anhilate Flowey overriding his save powers, then blast through Asgore and leave with the other 5 souls. The Genocide run was more fun for me than the Pacifist, which may be true for Undertale too because I'm a gameplay first type of person, but at least there I enjoyed the pacifist story too.

Lastly, the overall story was good. I enjoy that everyone (even Dalv) has a connection to Chujin, though I'm not sure I know where his hate towards humans came from. I might be forgetting something. (Oh, right. Kanako's friend was attacked by the blue soul. Eh, seems like a weak motive. Maybe Kanako being attacked would have made it more impactful and logical.)

Music was ok but it obviously can't reach the standard Toby Fox has set. I felt like there weren't enough motifs (if any) but I may have missed them.

Overall, this is too much of a fan-game and does not do enough of its own thing. I'm glad to have played it, but it just doesn't reach the expectations I had based on other people's comments. I'd recommend this to players who are starved for more undertale content if they are not satisfied with just Deltarune, or two people who had a big Undertale phase in the past and are nostalgic to have a fresh but familiar experience. If you played Undertale and thought it was just ok, skip this one. If you haven't played Undertale, go play that instead and come back to this in a few years.