The parties on the fringe usually criticize bs rather than the real problems, because they at most don't care of improving the EU.
In the best case when it doesn't work well it's a convenient target to blame (when it's their own fault).
In other cases they'd like the freedom to be *more* (much more) authoritarian than EU membership allows.
But currently let's be honest, most simply work for Putin or China and the aim is just to wreak havoc.