Perhaps you can elaborate on what was just said, by Roboson.
To answer this lets start with:
Imps are about as strong as the Unsouled, which are about as strong as your average human, about as smart as a human(which can't be said about the other two options, one of which hits "Swarm" insect level) and have skin about as tough as leather, they gain disadvantage in the height direction(They are only 2 feet tall) in exchange for their many benefits.
The Lich is a "Master of curses and some minor necromancy" that means he knows multiple debuff spells (exact debuffs will be learned if you take him) and has the ability to produce a few hundred skeletons and other minor undead outside of combat each era.
As discussed on discord, the lich is a support hero. it has debuffs for the enemy troops and provides a few extra units to us per round. While these could be nice, they lack a certain offensive ability. He can hinder enemy troops and give us some cannonfodder spookyscaryskellingtons, but wont win fights. Hes an accent piece, a guy which inst great on his own but needs others to shine, a behind the scenes string puller. In other words, he's a support.
We don't need a support right now, we need something that can win fights. The arch demon is a high damaging melee fighter which also buffs our units. Thats something thats always going to be useful and is going to give us an edge over the enemy. The lich is more situational, and doesn't seem to provide enough of one thing to straight up lead to victory (like the archdemon)
Archdemon: Damage, antitank, and buffs our units.
Lich: Provides some spare units, debuffs enemy units.
One makes us strong and is strong. One makes them weak and is weak. We should pick being strong over being weak.