I was in UCU (or AUT) BUT I moved a while ago to ATL: this was for a specific reason over a very political battle UCU continues to try to fight and promote, which is strongly motivated by the SWP element (who are very active in the senior UCU levels), and one that makes me deeply uncomfortable, as I find the premise on which this particular is based not only deeply political (as stated) but also fundamentally flawed, clear evidence of dual standards and, frankly, seriously troubling in terms of academic freedom and integrity. (Sorry - a bit opaque, but trying not to totally out myself!)
ATL are really helpful (though more geared towards FE than HE, they do have an HE "wing" and are much more "positive" with regards to the various challenges of the sector than UCU, whose default position seems to hinge on striking, and/or alienating our students) and did a great job for a senior colleague who was bullied out of her job. They aren't recognised by my institution in terms of pay negotiations, but as those apply across the university, this doesn't matter per se: you don't need to be a member of any union to benefit from a union-negotiated pay-rise.
UCU, by contrast, have just done a very shocking job for another colleague being investigated over a very serious issue: this colleague took advice at branch level, and followed it, but as it became clear that this issue was being taken very seriously, the branch started covering their backs and asked the region to step in, with the result that this colleague went to a key meeting with union representation from someone with whom s/he had not even had a chance to meet prior, much less brief. So clearly, it depends very much on your branch reps (this one in particular has been a union rep for over 40 years, though not always with UCU, having had a different career previously) and they will not necessarily have your back.
Clearly, I am only speaking from my own experience, and 'til the above situation, which is still rumbling on (but will likely lead to said colleague's dismissal, through no particular fault of his/her own, having ended up in this situation through having, quite properly, followed UCU's advice), I was only opposed to them based on their stance on the issues I mentioned at the start: because of the issues I work on, it simply wasn't possible for me to continue to be a member of the union that practised this kind of approach. However, following said colleague's experience, I am now rather prejudiced against their support if you ended up on a serious disciplinary charge.
Like I said, ATL are more geared towards teaching and FE, but I have found them very helpful whenever I have needed to contact them. They helped me resolve an issue when I was being bullied by a member of senior management, and have an excellent series of confidential telephone numbers for assistance on various things. Clearly, though, a lot of people aren't comfortable with being represented by anyone other than UCU in our sector, and I respect that also: but they aren't for me.