I'm a dom, so I'm sure I'm not the right person to answer this, but I'll give it a try.
If a woman had turned to BDSM as a dominant as a way of asserting control and expressing power as a way to make up for her past, where she had been denied that power by abuse, then I'd caution that "DIY mental therapy" like that might not be the best approach. I'd be the first to admit that I'm not an expert, however.
If she had been in an abusive relationship, however, then I'd almost suggest that that would heighten her sensitivity to issues surrounding partner abuse. I would, admittedly without knowing anything about the situation, be less inclined to believe that she'd be out for revenge against a proxy for her abuser than that she'd be sensitive to insuring that she wouldn't hurt anyone the way she had been hurt.
If you're talking about your mistress, then you likely know better than any of us. If she's still dealing with issues from her past abuse, then she probably would benefit from talking to a professional. Other than that, if you are satisfied with the relationship dynamic - particularly that you feel safe while she is exerting physical control - then that's probably the only other real worry, and it's not particularly different in this case than any other.
I hope I haven't put my foot in my mouth.
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http://www.amazon.com/The-Society-ebook/dp/B00BPF9U2I