I'll chime in and say that Perma-deaths being an actual flaw is an execution thing and all that.
FE games, at least more modern ones, handle it well for example (especially since the addition of Casual mode, for entry level players. With the exception of "Bullshit Crits", FE games give you a sense of control. You can see enemy stats, see who will live, position characters to avoid it, heck, you can even see enemies Attack Radius, so you legitimately find out safe spots, and not get gimped unexpected terrain nonsense. It's not perfect, but often when you die in FE, it's your fault, not the game's; I say "often" because, again, there are some random factors that you can't help. I suppose you could argue "You shouldn't have changed that 1% Crit with someone who couldn't survive the blow!" but then there are cases where no one CAN legitimately survive it even at full HP (this is a rare, exceptional circumstance, I'm aware.)
X-Com, as noted, is another good case. Since everyone is a generic of sorts, that means every character you lose is replace-able to some degree. Losing a high level unit of course sucks, but not only are higher level units more durable in nature, there's nothing inherently special about them (besides Psychic y/n?), so it's lost ground that can be made up. It's a set back, depending on how many units you have, but a recover-able one.
Then we have FFT and TOPSP, where perma-death's exist, but there's a grace phase. Given the nature of the games, that's fine; sometimes you can't avoid taking loads of damage, and trying to 2nd guess the AI without being a complete nerd at the game isn't realistic, so at least when you hit 0 HP, you can still save the unit via Resurrection (also resets the gauge) or finishing the fight fast.
Tactics Ogre (original) and Hoshi, meanwhile? Examples of games pulling it off badly, and it detracting from the game. No real way to prevent it, if enemies bum rush one character, they can be dead, lots of praying the RNG doesn't screw you over, etc. They're clear cases where Perma-death was a very bad design decision (TO at least acknowledged it with the remake, as addressed above.)