Re: Apostacy on mindless lifeless constructs?
 

In my tests of the spell it once took 11 castings to net me an MR 10 sacred unit. That might not be so bad in the early game, but consider what other spells those H3 mages could have cast instead of apostacy later in the game.

As for the stealing of other peoples mages, well they probably have a lot better mr than 10.

Re: Apostacy on mindless lifeless constructs?
 

Was your test in CBM or vanilla?

Re: Apostacy on mindless lifeless constructs?
 

CBM, of course. And other MR resist spells that I tested at the same time worked much more often.