So

1. Magic buildings have cooldown on their skills, so the extra mana from mana generator is useless to them
2. This whole race was not balanced properly even in WC3 and now its visibly weaker than any other, the reason being their unit cost-efficiency:
Water elemental - full cost is 570 gold, strong unit but doesnt do a lot of dmg. For its price...its ok, not more.
Lighting - costs 510 gold...provides slow 70 aoe dmg, compared to other single units for 500 gold or for several units of that price - weak. His passive is good, but if theese guys enter the melee - first line is done for and they will die too soon
Fire - 240 gold, 45 dmg and nothing else, super weak. Upgraded guy is good, but total price of 480 gold makes him late for the show, so theres already someone to kill him fast enough before he can proc his passives
Earth - straightforward tank unit, does his job, but nothing more

And now compare this deck to Nature -
Splitting-webbing spiders
Healers
Buffers-summoners
Ever-regenerating bashing cleaving bear (OP btw right now)
Drake sux ok
Giant...like earth elemental, but bigger, so he's the boss

And now compare this deck to Chaos -
Quill mass dmg backfire aura for 400g
Tough angry guy with cleave and cool buff for 380g
Miss Makes them miss their target, not bad
Blood friend, bro, divine-lifesteal
Drake sux ok
Infernal...like Giant, but angrier (could beat Anything on Elemental side for 500g)

You see my point bruh?)

It's not only about facial price of units. In the balance there is also the snowball effect you can obtain with the spliting and illusion units.

"Obelisk of Element" is listed as killing building and not support building.