and "situationally" in this case means "literally while doing anything" because you are always going to be in a state of moving or having just killed something. There's absolutely nothing about it that it's a sidegrade in the slightest sense unless the damage reduction were to be tripled or something.
If you're reliably finding someone to kill every three turns in a boss fight, lemme know how. Unless you're killing all the bosses in three turns, somehow, at which point you got even better weapon luck on your Cursed than I did, and I think that might be the best weapon luck I've ever gotten.
I suppose you could rack up movement speed and try to intelligently dash around between the enemy's turns to keep up allres, but that's only available either pretty late in the game, or when using Rampage(at which point you should be using your turns hitting people, in general, though I'd imagine there's some exceptions.). I also don't know how well that actually works in practice, either.
3 turns is longer than it really is with rampage.
Also, which boss fights would it matter in? Let's see, going by mid-game:
1) Master. Pretty self-explanatory what with him being a necromancer.
2) Orc Ambush. Ditto except replace necromancer except "you are literally surrounded by orcs. everywhere. you are in orc hell."
3) Orc Breeding Pits. Not a short supply of enemies here, and even then the boss isn't very threatening.
4) Underwater cave. Slasul has friends with him and also isn't threatening for a cursed; ditto the dragon.
5) Vor armoury. Targets abound everywhere, Gnarg coincidentally isn't very hard on a cursed either.
6) Vor Pride. Well, you're in a pride, and also Vor is going to crap himself the instant you get into melee range because he will explode.
7) Grushnak Pride. Ditto and cursed don't have a hard time with Grushnak.
Gorbat pride. Finally, a boss that can mess you up and is kinda scary in general. Except he summons a lot so anything he decides to drop on your head if you need to back off and heal works out perfectly.
9) Rak'shor pride. Also pretty self-explanatory what with the necros everywhere.
10) Eruan/Charred Scar... well, who cares.
11) High Peak. Targets abound, don't have to fight stair bosses (and regardless of whether you do or don't your investment or lack of investment into curses won't make or break the fight if a fight isn't perfectly set up to make use of shrouds).
12) Elendar/Argoniel. Cursed thrash them pretty easily nowadays so it's mostly moot but I guess if you were super worried you could leave an easier portal open? And if your gear is bad then you'll spend enough time dancing around the top/bottom pillars for the extra resists anyway while stalling to heal or movement/rampage across the room.
Basically, boss fights would be a much more pressing point if cursed didn't basically style on 99% of them and some of them involve movement anyway (vor deciding he doesn't want any of the crazy screaming dude's crap and backing away, ditto rak'shor, grushnak's knockback, repositioning during Gorbat if he decides to manhandle you, Elendar's knockback if he gets coy as well as his habit of running away because he also is not down with the crazy screaming dude, etcetc) so you tend to constantly be benefiting from at least half of shrouds as it is.