Just played it. So I'm going to try to give you reasons NOT to buy it, without being unfair.
Playing normally on the hardest difficulty (not trying to speed-run or doing a no-gold run or something), it's a nice challenge but nothing too hard, especially considering there's no permadeath - just a very meager stat loss. Which seemed to be pretty much a ground-level design choice, for two reasons:
First, as you get further along in the game, enemies start to have somewhere around a 20-40% crit chance on average, and since luck got changed from the crit defense to the crit chance stat, there's no way to protect your guys from eating a couple of crits each mission - other than to make sure they're only hitting dudes with high defenses or giant health pools (crits are, mercifully, 200% damage rather than 300%).
one autist who complained that there is randomness in the damage calculation, so you can't plan the entire enemy turn ahead like in Fire Emblem.
It becomes increasing true as the game continues. This goes for the player too, of course, and since there's an abundance of "chance to proc for extra damage skills" that makes the game feel far less deterministic. Unlike hit chance, they don't scale with any stats, either, so it'll always be a 30% chance to deal 30% bonus damage, or 10% for 50%, or whatever.
Second is the result of the weapon-upgrading system. Instead of having weapons that you equip and pass around, each character has 4 "weapon profiles" - Power, Crit chance, Accuracy, and Balanced(Speed). These are always available to every character, but each individual profile has to be upgraded on its own with tokens you buy or are sometimes dropped. If guess they figured everyone would reload if a character they had invested in heavily got killed, because it would really put you back. Sadly, this means there really aren't any weapons with unique properties, and single units can't use more than one damage type. Even worse, if you upgrade a weapon even just a couple of times, it will outdo other profiles in what they're supposed to be good at, meaning it's the most economical to just upgrade a single weapon and use it every time for the majority of the game.
Compared to fire emblem, the damage type vs armor system is kind of a mess. Now you can get matchups where both people are getting bonuses against each other, or both are doing basically nothing, but it's not worth it. You still get the dynamics of arcane magic being good against armor, swords being good against mages, hammers being unable to hit anything that can move, but in a far less elegant way than that being represented by speed, defense, resistance, and luck - all of which are still in the game. It's not always intuitive, either: for some reason lightning is ineffective against plate armor, projectile (bow and arrow) is a different damage type than melee piercing and is ineffective against robe-clad wizards, to name a few examples. Yet the result of all this is a game that plays basically identically the GBA fire emblem games, except sometimes it's ideal to send your unarmored lightning mage to fight their leather-clad archer.
There's also a "Mastery" stat that either increases your matchup bonus or decreases the enemy's if they are effective against you. Looks like they wanted to mix things up just for the sake of it, but I think this stat was probably the best of the new ideas and has potential in the traditional weapon-triangle. Sadly, in the current system, all it does is make it more impossible to get an idea of how much damage you will deal and take.
The numbers also start bigger and grow more, making it more "RPG". You also get these artifacts that you can equip, one per character -
Balance may vary, but those are where most of the fun is at when it comes to builds. Most characters, either by their personal skill or stat growths, are pretty much only fit for one or two of the advancements and will severely underperform otherwise.
As for aesthetics, the sprite-work is generally pretty good, with a few exceptions (archers arms stretch to about twice their normal length when firing, for some reason...). Surprisingly, there's no male or female body types for sprites, either, but you stop noticing after a while.
The writing is absolute garbage. Plot, characterization, world building - terrible, all of it. Absolute shit. Music is also bad, unimaginative and grates on you. Art is ok, nothing special.
Boot is pretty much correct, though he's going pretty easy on the music. There are about two good songs in the entire game, and you barely hear either of them. As for the rest, it's so awful it becomes significantly damaging to the experience as a whole. It's almost as if the choice of music is intentionally bad as a joke or something - even just searching "free fantasy battle music" and picking at random should've yielded a better selection. This does not improve as the game goes on.
Basically, it's exactly what you think it is: a decent fire emblem knockoff, not a masterpiece that eclipses the franchise it's based off of. The gameplay is pretty fun, there's a few bad maps front-loaded at the beginning of the game, but they (mostly) get much better. If you're at the point where you're considering playing or already have played romhacks or srpg maker games, you'll enjoy it, despite the unnecessary clumsiness of the systems.
Is it worse than 'Fell Seal: Arbiter's Mark'? Because that game is fucking woeful in the aspects you mentioned (and even worse in the art department), aside from the customisable difficulty which is decent.
It's not quite as bad as fell seal, but that's not a particularly high bar to clear. Dark Deity is certainly not a game you'll want to play for the parts where characters are talking.