To some degree it's customization, but I don't think units should be entirely inflexible or anything, as much as I prefer units to have a set role or style that you have to work around. What I really dislike is the constant busy-work that goes into building units, and how the game keeps shoving these mechanics in your face.
FE11 gave us reclassing, which changed up your units' stats and growth rates. By and large you really didn't have to do this, and leveling was limited, so the benefits of doing so were pretty small outside of a few fringe cases. And on top of this, you could only have a finite number of each class. Ultimately, you weren't encouraged to be constantly reclassing and building units by this method, because it wouldn't be super effective for more than a few units at a time. Though the endgame results didn't really deter you from just making as many high level Dracoknights as you were allowed and just rushing, which hurts the endgame of FE11 and 12 in my mind.
FE13 started snowballing things out of control, and it's where I find that the mechanics became incredibly intrusive to the core gameplay. Beyond reclassing, skills were now tied to classes. And instead of skills being permanently tied to classes or units like in FE4/5, or a finite resource that were mostly on a unit-by-unit basis like FE9/10, Awakening allowed you to take skills from classes and bring them to other classes. And reclassing only changed your stats based on the bases of the classes you change between. The game now incentivizes and rewards you for constantly changing the classes of your units and getting new skills for them. To add on to this, the way certain skills worked(Like Armsthrift), things like forging now became much more important than it was in previous games. Now you're going around scrounging for weapons to forge for each character, which is just a huge random chance deal, and switching everyone to the best classes available with the best skills available. In the base game you can mostly avoid this(Though it doesn't stop the game from shoving second seals in your face constantly), but it's pretty much a necessity for the DLC, and changes the whole feel of the game when you prepare for this by making as many Armsthrift Sorcerors/Dark Fliers with forged Aversa's Night as possible, or any other combo of insanely busted classes, weapons and skills. And there's not much of an alternative to doing this, which is where it starts hurting the game in my mind. You ignore these mechanics only at a detriment to yourself.
Fates stripped back the insane character building by limiting levels, reclassing potential and increasing the difficulty of grinding, but it, instead, adds MyCastle. What used to be a pretty straightforward SRPG series with a structured and deliberately paced campaign is now mostly an RPG with some strategy elements where you're shoved into an insanely grindy village/farm building sim every 30 minutes, with some weirdo mechanics like a lottery sprinkled. You gotta build your MyCastle to maximize your material yields, and you have to build around certain facilities in the MyCastle, but you can only get some materials and facilities in a given playthrough, so you have to tediously visit other MyCastles if you want to get resources you don't have, buy weapons you can't get at your own Castle, or get skills you've locked your own characters out of. And the game keeps encouraging this by making you build up points that you can get through online interactions which then unlock rewards in your own game, some of which are only obtainable through this method, and can require thousands and thousands of points which you build up at a pitiful rate. And then, if you decide you want to play one of the other routes, you have to do this all over again.
Which, again, wouldn't be too big of an issue if the game gave you incentives to not do this. But it makes MyCastle a core mechanic, and constantly makes you go back. It's even harder to ignore than reclassing in Awakening. I don't think MyCastle would have gotten hacked to hell and back if it wasn't so essential for certain aspects of the game. If it was more ignorable, or if it was more fun, I don't think hacking would have been so prevalent, and hacked Castles wouldn't be so popular for visitors.
And Three Houses seems to be diving headfirst into micromanaging and busywork again, possibly even more than what Fates did.