I will say two things.
1. Is that the claim that all doctors, even the significant of majority of doctors, are worthy of our blind trust in our treatment, is complete rubbish.
The transgender community, including this forum, is littered with countless examples of doctor incompetence, and it seems clear to me the only reason people make this assertion is to discourage self medication out of a paranoid fear of legal reprecussions.
Now, having said that, a lot of doctors ARE competent, a lot of forum posts are completely incompetent, and I will be the first to say I for one am not competent enough to judge if provera is a more powerful anti-androgen than spironolactone. I can absolutely say that it is used for exactly that goal in chemical castration (which is basically a massive suppression of testosterone).
I can also say it's usage for this purpose is uncommon, and that spironolactone and cyproterone acetate are more the norm.
I can say that unlike more typical and specific androgen antagonists, I can't find much information in a quick google about how provera works at being an anti-androgen.
EDIT: One other thing, as far as I'm aware, none of the estrogen replacement pills are unusually hard on the liver, neither is spironolactone. Cyprotone on the other hand is.
Disclaimer: This post should not be taken to be an avocation that people ignore their doctors advice, I believe everyone should arm themselves with as much knowledge as possible and base their choices on that.