Turnout in PCS elections is so comically low that I'd forgotten Serwotka was no longer GS.
PCS has a concentrated form of the problem you get in some other public sector unions, where there are some very good people at branch level, but the leadership are fixated on their image as a Very Left Wing Union, and they're more interested in performatively supporting the Current Thing than doing the basic stuff around pay and conditions.
I'm entitled to join FDA, and sometimes think about it, but I'm not convinced FDA has a membership pitch beyond "we're not as barking as PCS".