I agree Conscript that the CPUSA isn't the best example of other "MLers" but it is only a more extreme example. The PSL hypes Charles Barron and his Freedom Party, but that Party is only a out-group of the Democratic Party (Barron will do stuff like "endorse" the PSL's NYC mayoral candidate at a PSL event, but on his website and such tell people to vote for the Democrat). In other words, they aren't "uncritical" and do not just say "vote Democrat" but this is just one instance of how they evade a clear fight for working class political independence.
You say the PSL doesn't play a class collaborationist role in the unions. Where is your evidence of this? Where is their public record of union work? The WWP certainly plays such a role. Not as the CPUSA does, no, again not that extreme and explicit but just last year WWP played a big role in my local's "fight" against layoffs (in other words the WWP helped them "look" militant while avoiding an actual mass struggle). Do you really think the PSL functions differently than that or do you have any evidence that they do?
Further the "Leninist left" (International Socialist Organization, Solidarity, the
Progressive Labor Party, and the Freedom Road Socialist Organization) did work together on union work in Chicago--and fucked it up:
http://www.lrp-cofi.org/pdf/ctu_052111.pdf.