You know... I really could think of a lot of storytelling aspects of slaves... but Bioware's target audience would be outraged.
Real slaves didn't revolt at the drop of a hat... some of them actually loved their positions in life - while the vast majority simply accepted it - and that's hard for modern people to accept.
Honestly... having a slave companion would be amazing I feel.
Oh believe me I know, or rather I have read what other people have to say on the matter...
Its a old story, it goes many centuries at the very least in a lot cases. The Persian Mamluks were highly regarded soldiers and scouts and they were enslaved under the Fatimids and like minded cultures in the Middle East, then came the crusades, the rest is history but the point stands for several generations you had a entire culture based around mercenary service that would resemble indentured servitude. Stuff like that doesn't exist in DA though.
I think the fact that the Rite of Annulment was created and has been used several times in the past few centuries is enough evidence that the system doesn't work "well enough." It's personal opinion where the line of "enough" is of course, so I doubt this line of conversation would be useful.
Pretty much, I'd say that given its been used less then twenty times in a thousand years is pretty solid backing for the success rating, also if you actually read some of the accounts revolving around those rites, they are generally justified. Heck in one example in DAI itself, you hear about a annulment where every single mage in the circle was killed because every single one of them fell to possession. Like Broken Circle but worse.