Imps - 3 of them, are beyond deadly for 2 Level 1 characters in D&D. 3 intellect devourers are beyond deadly for 2 level 1 or 2 characters.
A party of 5 or 6 level 1 characters have a much better chance at 3 imps, and it's more balanced. Likewise, a party of 5 or 6 at level 1 or 2 have a much better chance against 3 intellect devourers.
Yes. The ONLY reason these encounters aren't impossible is because the monsters are nerfed - severely.
All Larian has to do to balance these encounters is allow for party of 4 custom characters, increase the party size max to 6, and implement actual 5e monster stats.
There's nothing wrong with the encounters. It's the party size and the fact that they aren't ACTUAL D&D monsters. They're baby thug monsters that have been given the appearance and names of really tough monsters.