Uranium is a Pokémon game, so it defaults to being at least somewhat good. Its issues come both from staying too faithful to the official games and from deviating too far.

There was really no need to keep HMs in the game, especially since there's an NPC who gives you items to replace them. Even the official series dropped them in Sun and Moon shortly after Uranium's release. Even worse, you don't even get Fly until after the eighth gym. The game also has trade evolutions, which is the worst thing you can add with online functionality. Even Gamefreak has started putting in in-game trades for some of the trade evos.

Uranium, being a fangame, does not have to be family-friendly like the official games are. Uranium's story was a shot at a Pokémon story with darker and more mature themes. What they did with this was make an edgy antagonist who feels like a cartoon supervillain that we are supposed to take seriously. For most of the game, the villain's motives are "I suffered and now everyone else has to" and it's not until the end of the game that we're hit with the Tragic Backstory™️ that tries to redeem the villain in a single lore dump.

Uranium's fakemon have a decent ratio of good to bad, similar to that of an official game. They can range from really good designs (Gellin is cooler than any official grass type) to ones that I question how they made it past the drawing board (Barewl and Titanice). A lot of cute, well-designed first stages evolve into somewhat disappointing final forms (Lunapup, Minyan, Selkid).

Speaking of fakemon, the game is incredibly unbalanced, and the devs knew it, too. Pokemon like Nucleon, Inflagetah and Harptera are all completely broken and the game takes advantage of it once you get into the upper levels (Theo's hyper voice Nucleon, Tiko's choice-speced contrary overheat Chimiconda and Mega Inflagetah, a Pokemon that was overpowered before mega evolving).

The level curve is obnoxious, sometimes the wild pokémon will be ten or more levels lower than the trainer pokémon, and that gap is even worse for gym leaders. The best grinding sources (EV ninjas and Championship) aren't even available until you're done with all eight gyms.

Despite my paragraphs of complaints, It is still a Pokémon game and I did have fun with it. Really it just needs some polish and a few minor tweaks (or a bit of rewriting to make the villain less edgy) and it would be a much better experience. I'd recommend it with a warning that it has some issues.