I think one issue, maybe the biggest issue, is how the magic slavery in itself works, okay, employers have obligation of treating the slave well, okay, fair, but how will people know the abuse happened? Is there like a magic to know if people speak the truth? Does the magic slavery spell "marks" the abusers in some ways? Or make the collar shine red as if saying "hey this slave was abused"? Anything really?
I feel that that japanese writers half-assed magic slavery too much, especially because it involves literally fucking magic, they want to portray it as good but they never really develop the system to show how it can't be abused because magic will cause consequences, so it end up mostly as lip service.
"Slavery can be good because I said so, look, I portrayed it as good, don't question the details though!"
There was countless cases of slavery being "good" in human story, but I feel that when it involves magic author has literally no excuse to at least show a proper magic system, with punishment and such if slave is abused, if they really want people to buy that it's actually completely good.
I still think part of the issue is cultural differences, federalism still too strong in Japanese culture, so they don't see slavery as most westerners.