The Fell Xenologue definitely has it's issues in terms of difficulty scaling and the baffling way it's set levels work (Characters being at a set level on a per map basis, but never getting downgraded in classes; meaning that on map 1 you'd have a level 15 myrmidon Lapis and a level 15 swordmaster Kagetsu, like c'mon lmao) so I would definitely recommend just playing it on Normal There's some fun ideas with it's maps, but the tools you're given really don't feel up to par in a lot of situations so playing through it on higher difficulties is really not worth the headache, unfortunately.
Luckily, it's difficulty scaling is seperate from the main game; you can blaze through it on Normal and still appreciate the regular playthrough in your difficulty of choice, so that's a relief.

That being said, I genuinely appreciate the story a lot and Nel, Nil and the Four Winds are some of my favorite characters in both this game and the series as a whole, so it's peak actually!

Madeline and Gregory are kinda average as units, though the latter is the uncontested king of the Olwen ring but Zelestia, Nel and Nil are all really good and fun to use, especially the twins considering they interact with Emblems like no other as Dragon units.

Haven't dabbled with the new classes much but Enchanter's gonna be really interesting - Weapon Surge opens up a lot of new tactics especially for goofy challenge runs where you stick to one class for the entire team, something I'm quite fond of myself. Definitely wanna try an all Arts run with it's +5 MT to an Arts weapon of choice mechanic, someday.

So yeah, the Xenologue working the way it does is a bit of a let-down, but it's story and all the supports the DLC characters get are fantastic and they definitely made replaying the game worthwhile.

Whether it's worth the price is up to you, but as someone who enjoyed the main game a lot - this was no exception despite it's flaws.