If it is a traditional trade union that you find in the West, then it leads to reformism and compromise, as you pointed out, as well as monopoly-style cartels on labour forces. These so called "leftist union leaders" tend to officially only care about their union members and privately their next paycheck. Obviously they were founded in order to counter capitalism in a constructive way, and we must not forget it is largely thanks to them that worker conditions in the West and elsewhere are better, but I find they have been watered down to impotent, loud, annoying pseudo-trotskyists who are really just part of a bourgeois establishment.
I'm not speaking about all trade unions btw, and I do agree with the principle, but I don't listen to their so-called revolutionary talk, as the bourgeois government tends to just goad them with privileges and reforms - which makes them collaborators IMHO.
As for (anarcho-)syndicalism, mutualism, etc.. as an ideology, which i think is your main point. Well, I have no issues with it, and I believe most non-MLs on here don't either. The MLs tend to oppose it since they believe in one single vanguard movement instead of a multitude of loosely allied organisations to represent the revolutionary movement. When you look at the excessive syndicalism in Republican Spain during the Spanish Civil War, it pretty much cost the Republican side victory, so despite the clear authoritarian agenda, the MLs have a point.
Syndicalism also leads to tribalism, which itself would probably bring us back to capitalism again (the two complement each other rather well). A lot of mutualists are not for the abolition of private property for example, just the sharing of it within their "group".








