Why are candidates the canaries? Because they are the weakest members of our communities. Generally, they know few other members of the Jewish community, they may not know any other rabbis, they likely have weak ties with their own birth family, and they generally have little experience with Judaism. Anecdotal evidence suggests that conversion candidates have suffered abusive relationships more than average, whether parental, sexual, or partner abuse. That combines to create a fertile ground for abuse. (Thankfully, the internet has evened the odds significantly.)
A) They may not know they are being taken advantage of,
B) They may not know that a rabbi is violating the halacha,
C) They may not have Jewish friends they are comfortable discussing sensitive issues, and thus, don't have another Jew to tell them something is wrong, and
D) Even if they know something is wrong, who would they tell?
In other words, conversion candidates are the weak gazelles of our community, the ones who are easy to pick off. This is why the Torah commands that we protect the ger: the ger doesn't have a portion of the land in Israel and doesn't have a tribal affiliation. There is a potential for abuse and taking advantage of the ger. Today, that risk falls on the conversion candidates.
Rabbinic abuse of conversion candidates is rare. It is certainly rarer than you would expect, by a long shot! If this were a secular enterprise, I think the rates would be astronomical compared to what they are in our community. But that doesn't mean it doesn't exist. Most abuses are financial or pure power trips. There have been sexual abuse cases sporadically, the most infamous is the Tropper case (See, for example, this post.) Note that there are rumors Tropper is back in business again with a new group, but keeping his name off the paperwork. If you meet this man, run away!
So what's my point here? If something seems wrong to you, it might just be wrong. Halacha requires you to judge favorably, but it doesn't require you to forget. If you see something that strikes you as a red flag (and there is an innocent interpretation), give them the benefit of the doubt and move on. But remember that red flag and keep your eyes open. Watch for other red flags.
Of course, if something is beyond the pale bad or crazy, don't be afraid to call it like it is.
There's another prong to cultivate in order to protect yourself. Cultivate your relationships with other Jews. Find a mentor, find an "adoptive" family, make close friends. Get active in the online Jewish community. These are the people who can be your sounding board. They can help you figure out whether the behavior was harmful or harmless.
Learn about rabbinic organizations. Learn who are the rabbis "over" your rabbi and beit din. If, Gd forbid, you have to make a call, these are who you would call. Of course, it's possible you could be wrong (and it's also possible that a bad rabbi could weasel out of valid allegations). That is a risk. Hopefully it is a risk you will never have to face.
What's the risk on the other side? There are mentally unbalanced people who consider conversion. They generally don't get very far in the conversion process, but they might stick around in the community for years. People with victim complexes, people with anger issues, people with serious issues with authority, people with skewed visions of reality, pathological liars, attention-seekers. And these people can create false allegations either out of spite or a disconnect with reality. I wish that was a risk that rabbis never had to face. The moral of this story? Don't necessarily believe allegations you hear from someone else. Keep your eyes open, but also watch that person for warning signs that he and reality might not be on the same page.
(Of course, the current child sexual abuse issues highlight others in our community who are disenfranchised and abusers who have been protected.)