But if that's the case, why would they go for a "party initiative" in BG3?
Side initiative has several issues:
- Combat becomes much more volatile and heavily influenced by a single roll. Alpha striking and burst damage become a huge deal, and both groups can get blown up in the first turn with no chance to react. This only gets worse at higher levels.
- The volatility is more "balanced" on tabletop because there's a DM controlling the enemies, so the extra lethality goes both ways. This can lead to cheap PC deaths or TPKs due to a single initiative roll, which is one of the reasons why almost nobody uses it. However, there's no DM in a CRPG, so in BG3 it's strictly a buff for players (especially savescummers), since the AI will never exploit it as much or as well as the players.
- Several 5E subclasses have initiative modifiers as one of their main benefits (and the Bard class has a bonus as well). I don't know how Larian is handling the initiative roll (How did the intellect devourers get 41 while the PCs got 18? And some previews say it has modifiers, others don't mention it), but such modifiers would have to be replaced, become irrelevant, or matter 10x more. Unless they don't implement these subclasses at all, of course. But that would be a shame, since they happen to be some of the more interesting archetypes for their respective classes.
The only benefit is that it's faster and easier for the DM to run. This doesn't apply to CRPGs, but it's still slightly faster there. As each side acts together, players don't get to think about or reconsider their decisions based on how each creature's turn went down. It could also counter/mitigate overpowered initiative modifiers and things like that, but that's not an issue in 5E.
So why did Larian go with side initiative? In addition to being slightly faster, my guess would be:
- Player party becomes more powerful, which seems to be a pattern across the changes they've made to the ruleset.
- It makes the Larian gimmicks with environmental effects and other abilities much more reliable to pull off. With standard initiative, such plans could be interrupted or ruined by enemy turns.
- Having all enemies act together while having to wait to react can make the encounter feel more dangerous, even though the system is better for the PCs. The psychological effect should be stronger on casual players, who are much more easily annoyed by things like missing or getting hit several times in a row.
- Players don't have to wait as long to see how their decisions play out, so they'll have to spend less time before deciding to reload.
(I'm not defending their decision, in case that wasn't clear. I think side initiative is terrible, but it seems Larian has a certain type of experience and challenge in mind, and that is a way to force/promote it.)