I used to belong to the AUT (predecessor to UCU) and left because it was absolutely useless. Every time there was a vote for the National Executive at least half the candidates would start their spiel in the election booklet 'I belong to UCU Left ...' and what followed was generally a long diatribe about boycotting Israel and so on. I belonged to a union because I wanted support in the event of any problems at work and I wanted them to negotiate better salaries, pension and working conditions for the membership. I made a point of voting for candidates who focussed on that, and the UCU Left bloc were automatically discounted for that reason.
From what I hear about trades unions these days they seem to be stuck in the same mindset that the Corbynites and Momentum had in Labour before the 2019 election. They don't want a broad membership and a focus on practical, achievable goals. They want only the ideologically pure and they are never happier than when they're passing motions of condemnation about areas that are nothing to do with them.
For UCU in particular to be telling the world that Professor Hilary Cass's report is substandard is just gutting. University lecturers and researchers with no clue about medical research and ethics.
Frankly I hope they go bust and then employees could start again and create new apolitical unions that actually do what the unions were set up to do in the first place.