Any company is free to hire whoever it cares at its own discretion, and in most (49/50) US states fire them without cause; perhaps due diligence is required of companies that are unionized to ensure that they are investing wisely in the labor they pay for, rather than accepting that all labor must be paid less on the argument that maybe it is of poorer quality than desired.

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.