If you work in a unionized workplace and have complaints about a coworkers capability, your complaints should first be heard by your union because your union is the arbiter of your labor force, as per the contract you sign with said union.
Guilds were (and in some non-US places still are) a solution to this issue, in which some level of competence must be demonstrated through time spent and qualifications earned to gain acceptance to a guild. Some unions in the US still practice this measure of trial for their members, but they are generally relegated to the skilled trades, and this isn't something that common labor unions do.
The question obviously being, what are you supposed to do if they fail to address it?
Suppose the union leadership brought in a bunch of their own incompetent cronies and is now making a hash of things, but you can't vote them out because those same cronies keep voting them in.
Also, how do you get to pick which union to be in? One can get certified even if you voted against it, or already exist at the company where you want to work.
Labor is not a homogenous block. A huge chunk of workers are lazy as fuck and only do the bare minimum; it's unfair for people who work hard that their compensation should be lowered just so the lazy ones can be paid more. And lowered it must be, because a company only can only afford a certain total amount of spending on wages, so if the shirkers must be paid more than the hard workers must be paid less. It's not exploitation to pay the bare minimum possible to someone who puts in the bare minimum of effort.