Noone goes mass air w/ Undead
you need to mix it up - frost wyrms are strong but it takes too long to tech up to wryms and if you are getting scouted constantly (or dont even know there is a detector in ur base) you can get caught teching (very awkward)

Lich is a great hero if you can use it...but ive only gone mass air with UD to counter vs mass Air..