The players won't have fun if they die or have to run from the last fight because an awful power limits most or all their options in combat.
I'm not convinced that's true.
Upthread, someone else was also talking about how players expect a "boss" fight, that Strahd cannot deliver... with the implication being that such a boss fight ought to be an epic slugfest.
But again, I just don't know that I buy that.
More story time, I guess. A few years back I played (not DMed) in a short-lived play-by-post Pathfinder game, the Rise of the Runelords path. I'm not a fan of PF, or most adventure paths, but I liked the folks I was playing with.
At the end of one of the first significant dungeons, at... level 2? I think? We found the big Boss of that dungeon: An Imp.
Or maybe a Quasit, I can't remember.
It trounced us. Nobody died... it didn't do enough damage, really. But we couldn't do anything to it... between flight, natural invisibility, and low-level spells to control us in various ways, we spent many, many rounds ineffectually trying to deal with the thing. I got in a brief grapple, the closest we got to taking it out, but then it hit me with a sleep spell and got away again.
Ultimately, we had to retreat with our tail between our legs. Anticlimactic? Perhaps. Frustrating? Definitely. But awful? Unfun? No way! That's the game! We left and plotted our revenge. Or would have, if the game hadn't died out a few weeks later.