I have known several places that were unionized.

the Union comes in talking about all the employees will gain from the union

They unionize, they get a 6% raise, but 8% goes to union dues so a net loss in pay of 2%, then the union protects the least productive members of staff, the productive employees get frustrated that they can not get indivual raises based on their own performance but get the exact same wage as Joe who does 1/2 the work, and is paid less than Jim who also does 1/2 work but has been on the job for 5 years more. (Unions are all about seniority not merit)

So 2 things happens, the good productive hard working employees move on to better paying jobs where their individuality is respected that could be a promotion to non-union job in that same company, or most likely to a non-union company.

The unproductive workers drag down the productivity of employer over all, increasing costs and lowering profitability.

Then after a enough time, profitability dips to the negative and the place goes under.