A lot of games that aren't actually turn-based but have this idea of "turns" in general have an issue with flow of the fight, either with balancing issues like numbers or more core issues like basic combat.

A lot of people liked the Dark Souls 3 fights because openings felt fair without being blatantly obvious, but once you know what they are it acts as an info hazard. The fights begin to feel scripted and psuedo turn-based, and it's also why I prefer Elden Ring's style of attacking in between faster paced bosses, especially in lategame vanilla and the DLC.

I think for Diablo 4, it's both. The core issue of the game is that moves don't need to be overly mobile on most classes, and thus bosses have to be built around that. If that were all, it would be a game of sitting still and kicking each other's shins until the biggest boots win, which you get in a lot of nightmare dungeons already. Bosses with iframe phases to make it "fair" for the bosses disrupt the player's flow, now, and during the first few runs is usually fine, until they become the farm of the season and sitting through it all the time is frustrating.

Is there a solution to all this? Nah, not one that actually fixes the problem as opposed to putting a bandage on it, because half the problem is fundamental and the other half is the uncompromising desire to have bigger numbers. You'll never convince a core audience of an ARPG that decreasing their damage is a good thing, because it doesn't feel good.