Senior/staff type engineers are not a union position so great people refuse promotions and responsibility because they don't want to leave the union. Thus they won't mentor juniors, and other things that you need great engineers for. (At least that is how the union people I work with in Europe are, there are other unions with different rules)
There is probably more.
Like siblings comments, I would be really interested to know to which country and union you are referring to. In France it's certainly not true (I have a very senior engineering role and I am in a union and it never was an issue, and anyway, the percentage of unionized engineers here is so low that even if that was true, it would hardly be noticeable)
Never heard of that, and I've worked in about 7 EU countries...
Never heard of that, nor of a union in tech. What part of Europe?
What union ? In which country ?
Which country is that in? Can you not offer them better conditions than the union? Are they forced to leave the union or just no longer required to be in it?