Adam I work at a university and was in the UCU for around 20 years. I'm certainly not someone who requires everyone to have the same views as me. Many of my lovely intelligent colleagues have pronouns on their emails and I think that's completely up to them. I am more than happy to respect people's preferred names, pronouns etc. However I left the UCU last year because:
i) They weren't just "not GC enough" (that would be fine), they actively spoke out against Kathleen Stock when she was bullied out for her GC beliefs. They have written posts such as this one which indicates they not only support trans rights (fine) but also are pro-self ID (not fine with me). I'm a hard working and respectful employee but the one thing I can imagine getting into trouble at work for would be if I expressed my GC beliefs and someone objected. I felt my union would not support me if this happened.
ii) I feel that their strike action in recent years has gone way over the top. I really appreciate them fighting for fair pay and pensions for us. But it used to be perhaps a couple of strike days per year if there was a pay dispute going on, then both sides would come to an agreement. Recently it's been weeks and weeks of strikes whilst refusing to give an inch. Then most recently they achieved a fantastic agreement on pensions which had been rumbling on for years, but instead of pausing and taking stock, they immediately went ahead with a marking and assessment boycott (one of the most damaging types of strike action for students, and a big ask of members) to cover the other ongoing non-urgent gripes. I thought this was wrong.
iii) In regards to changing things from the inside, it's a lovely thought, but in reality I don't have time or energy to be a union rep. I have an incredibly busy job plus young children and elderly parents. And if I did have time, imagine trying to persuade the UCU or any of these unions to a more centrist position on trans issues. You'd be attacked and vilified.
That's just my union experience.