Nope, that's just the nature of FE gameplay. There is so much going on in a map and maps go on for so long, that inevitably you're going to slip up and overextend a unit one more tile than you should have and then that unit gets eviscerated by multiple enemy units and killed, or one of your units is killed by a crit. So you wind up resetting. It is extremely difficult trying to progress through the game having lost a unit, because that is one less unit on the field (in a genre where your guys are almost always outnumbered), so that is a huge disadvantage, and can easily lead to softlocking your game because a level snowballs against you where eventually you just don't have enough guys to progress at all. Not literally impossible to beat, just practically impossible to beat in a timely manner. So the gameplay boils down to resetting which is painful if you are playing on console (or constantly making save states on emulator).
The implementation of "casual mode" in the 3DS Fire Emblem games was widely mocked, but I view it as an acknowledgement of the reality of FE game game design. The permadeath was an impediment to the series, not a well executed feature. Almost everyone resetted anyway, so allowing a unit to survive and just get taken out for this level allows the player to just move on with their life and save time. Permadeath only really works if you have a humongous pool of party members to draw from, and you can catch up replacement party members very quickly, so the death of a few party members isn't a game over (ie, Suikoden, Romancing Saga, etc).