I think the max level shards should give some scary demons
Some of them are alright, ignoring for now the cyndrum demon powerhouse (3k hp army — hello).
Ancient grave elementals
Too slow to travel with my armies, but they performed admirably as garrison supplements, defending against wondering wildland mobs and an occasional hero-less army that managed to slip past the front lines. They have something like 80hp to start, and regen + harbinger, so they are really quite good on defense, even alone.
Ancient burning wraiths
With 60% fire attack, these were actually quite good against well-armored units, dealing decent damage where my best trained armies could only clink. They are very much glass cannons though, so tactical micro is not optional (isolate single enemy unit and focus to kill, avoid a hit at all costs). Could use a small attack buff (increase the fire component, not piercing, maybe 10->12, 14->16, 20->24?). Not so useful if you are fighting Altar 
Ancient mirror elementals
These guys are useless. Damage reflection is nice as a concept, but it is lost on a unit that dies in one hit (low hp and fire vulnerability to boot, in a world where every bandit and his dog have a flaming weapon). They could literally be glass cannons, except that I think to be classified as a cannon you need to at least deal some damage, and 12 blunt is pretty pathetic for mid-game. I used a few of them as suicide bombers to soften up enemy front lines. An expendable caster might be an interesting role to focus on. Throw in a handful of cool abilities like shatter [damage adjacent units when killed] and mirror world [spell akin to death ward but resurrect with 1 hp after the battle; maybe also dink max hp in the process — only mirror elementals can return from the mirror world unscathed — to prevent spamming on regular troops], and all of a sudden we have some unique strategies to play with. Also, these guys are missing slow ability from their ice elemental cousins, it would fit quite nicely with the above mix.
Ancient crow demons
Haven't used them (corrupted all my air shards due to lack of air caster). With thunderstrike, high init, and high moves, they seem well suited to pick off back line archers and escape unscathed, but I don't know if that works well in practice.
[edit]
Speaking of cyndrum demons, these seem to be designed as swarm units, so perhaps something can be done to prevent them from becoming super-tanks as they level? Starting hp is already pretty low, throw in -2 hp per level and vulnerability to cold and fire in addition to lightning, and I think we'd be good.