[Illuminated One]

Yeah, Astral is very strong but very expensive also.

Just consider that your 50 units against 40 enslaving mages example he was outnumbering you probably by 10:1 or more in terms of gold.

I don't think the enslave the whole army spell are really that much better than kill the whole army spells - because of the mr+4 tag they get mostly chaff, the elites can continue to fight for the enemy.
Consider trying to master enslave an army of demons (14 mr) buffed with army of lead or antimagic (so they have 18 mr).
With an s9 mage and max pen equipment (+5 - 4 master enslave penalty) that costs you incredible amounts of gems, and is paralyzed after spending 8 gems to cast the spell, you'd still loose the battle because the 2 or 3 devils that are enslaved make no difference.

Now consider blood which has cheap spells for destroying SCs with no way to defend against it (claws of cocytus or life for a life), kill armies (there's a spell that casts life drain on the whole battle field for example) or summon SCs and armies + the ability to generate income at will, and I'm not really sure that astral is so powerful in games without clams.