A couple of points of justification.

1.) Getting enslave to a high enough percent to take the edge without using skill learn in conjuction with certain quests that give skill learns is a wild time sink. Over two orcs I spent hours spamming it to have it go up a net of 2% and over entire lifetime of both chars it went up about 10% naturally. So after hundreds of hours I wouldn't have been able to take the edge without skill learn, learn rates got better recently but at least 4 other people had similar issues with getting the skill up so I am pretty sure I am not crazy (my bet would be low int plus nuking of skills based on level of enemy used against plus needing mobs to be pretty lower level then you to use it)

2.) Slave selection is rough from 35-40, like very slim pickings. Around 42 it opens up a bit more in terms of volume but even at 51 the geographical location of mobs you would want to enslave vs where you would want to use them mixed in with the mobility restrictions of having a slave can make it bothersome to roll around with one.

If you were planning on making it less good I think like a 20% higher chance of failing on non traditional races would be cool. Or phasing in additional races with level, so like add trolls at 40, and giants at 45.