Also, like previously said, intellect devourers are supposed to have an instant-kill combat ability, so even 1 of them is a walking would-be TPK for newbie players. And do we really want Us to be able to insta-kill commander Zhalk and take over his body in the tutorial? What about canon imps' immunity to damage from Scorching Ray and Poison Spray?
The suggestion was to NOT use inappropriate monsters, rather then use inappropriate monsters and make them too powerful for low level parties.
I am not well versed in DnD monster manual, nor 5e in particular, but there is inherent appeal in starting killing proverbial (or literal) rats and moving on to epic enemies after X hours of progress and gain in power. I get that Larian wasn't their rats to be COOL, but then it detracts from actually cool late game enemies.
