As a minority, you can't assume that a union rep is on your side. Especially if the complaint is against someone else at your company who is represented by the same union, if that person is more politically important (within the union) than you are, you're risking a lot - up to and including retaliatory termination of union membership[0] by speaking out. It's your word against theirs, and you're not the one hiring your representative, so you can't be sure that their incentive is to advocate your case to the bitter end, instead of to brush things under the rug.
If you want someone who has no conflict of interest to advocate your case, you need a lawyer, not HR or a union representative. At best, the latter will refer you to a lawyer (at which point they're not doing anything for you you couldn't do yourself). At worst, they will cost you your job, and possibly your career[1].
[0] Which, incidentally, is not protected by the same laws that protect retaliatory termination by employers.
[1] If you're in an industry that's represented by one single union, having your membership terminated means you should probably start looking for other career options.
The problem I'm talking relates to how people in the majority group can politically overpower those in the minority. "Making unions more democratic" doesn't fix that; democracy as a system is literally designed on the principle of the majority (or plurality), not the principle of the minority.
Just as an HR representative works for the company (not for you), a union representative serves the people who elected them as a group, not you individually. If you are a minority member, you can't assume that your representative has your best interests in mind. And, by definition, you don't have the political power to replace them.
