Just like the others entioned; the transformation is not really worth it
The only reason for transformation is just as a breeding grounds alternative with lower initial power cost (90% of the original unit's power flows into the void and the transformed unit e.g. abomination would only bind 165 power instead of 220, would then return 90% of the 165 into void). For your t4 either transform your creepers or just use a breeding ground, sunken temple is not worth it as transforming uses charges.
Beware, when transforming they will stay at the same percentage of health, meaning transforming e.g. your minions (for damage boost) into vileblood will make the unit start out with less health. Instead of transforming just use spells, they are just better (e.g. Lava Field for T2, Thunderstorm for T3).
In conclusion, the transformation is just a lackluster feature from the original devs, its only efficient use is as a breeding grounds substitute (tranformations will most likely be rebalanced in the future)