The main problem is the total lack of intra European trade unions.
There are a few existing ones, but I'm afraid that they are only european by name. I couldn't find any blanket legislation providing an equal right to negotiate and to stand in court if needed throughout all European member states for these unions.
In aviation, regarding businesses of Ryanair or Wizzair, etc., it would be impossible to strike for example in Poland and the same time to prevent the seats to be filled by Romanian or Hungarian pilots in the same time. Works vice versa unfortunately.
If we can get that part right (in a federal Europe would be rather easy) these companies would have a much narrower playing field when it comes to further degrading their workers' conditions and rights.
I stand to be corrected regarding all of the above of course.