>>536884578
Pathfinding wise, having larger groups is a mess; some dungeons need individual jumping, characters are blocked in tight spaces like in Act 3 sewer leves, and generally the game areas are designed for smaller groups:
that being said
I make a couple of small groups, move them around and can really exploit sneak attacks and such.
Also some dialogues scene may be a little bugged but IIRC there was a mod for that.