I can assume the intellect devourers are weaker young ones like the imps are Lesser Imps. They can be from the same hatchery on the ship or however they are created. There can be exceptions. I understand they want a cool minion monster for the mind flayers that is lore and challenge appropriate. The problem is when these exceptions become too common.
Bosses are obviously custom monsters and they have a lot of freedom with them. Even so there are guidelines and rules that should make sense. Amount of spell slots for casters etc.
What I don't like is completely changing existing monsters i.e. Phase Spiders. The poison pools are asinine as a concept alone (more arcade/platform game than RPG) and spiders spitting gallons of poison is another.
Stat blocks of monsters are really inconsistent in BG3 too. Some random player race NPC minions have insanely high stats and I wish that was more consistent.
