The two major "slurs" against converts are accusations that you "converted for marriage" or "bought your conversion." You rarely hear the second one, but it does come up. Ironically, the only accusations I know of were in the chareidi world (ironic since you'd think those groups would demand "higher standards" than the modern orthodox).
For the first time (to my knowledge) since this blog started in 2010, someone got here by Googling "fast conversion to Judaism." In my experience, many people think the conversion process will be fast and easy...until they actually read about it. That hope is quickly dashed against the rocks of reality.
But some people feel entitled. Those people dislike being treated like the rest of us and feel that standards are really just "suggestions." If you have the right connections and/or enough money, you can make anything happen. (This should not be confused with a "squeaky wheel gets the grease.") I expect these kinds of unethical demands are more common in "rich" cities like Los Angeles and New York City, and in those who are related to shul leadership. There's nothing like your shul board or your major donor demanding a conversion, when denial or delay could jeopardize your job - so it's not always the rabbi's "fault." That is a very complex situation.
On the other side, you have people who feel they have to resort to distasteful methods to accomplish what they believe is right in a corrupt system. For example, the recent controversy of the woman who was converted as an infant 35 years ago and had her conversion rejected by the Israeli Rabbinate when she sought to get married in Israel. She admits, but will not identify, that her situation was "resolved" once "powerful connections" stepped in with protectsia (special influence that is possibly as mafia-sounding as it sounds like). Is this wrong or right? I don't know what to tell you.
With rabbis being human, I'm sure there are rabbis who have a price. I personally don't know of any (and please don't post your accusations here; those comments will not be approved). There are certainly rabbis who reject conversion candidates for corrupt reasons, more than accepting them. There is unfortunately a history of crooked rabbis in conversion; Leib Tropper being the foremost example of recent memory.
If you're here to covert seriously, be aware of corruption red flags, and get out while the getting's still good. If something feels wrong about a request for money or to see you privately, trust your gut and review the facts with someone removed from the situation. Give the benefit of the doubt the first time (and maybe a second if it's truly unclear), but eventually you'll either have to speak with someone in authority in the community or go to another community. If you acquiesce to a corrupt rabbi, you will also fall when he is exposed. And Gd-willing, he will be exposed.
It is a sad truth, but you may be thrown under the bus no matter how you deal with a corrupt (or mentally unbalanced) rabbi. But thankfully, I am confident that you'll probably never have to deal with something like this. However, we should do our best to take seriously those who believe they have been wronged. And to shun and ex-communicate rabbis who abuse conversion for their selfish goals. If you are in a bad situation, get help.