Well, my friends and I have never even been to the Union building. We don't know what it is there for or where it really is, to be frank. But it's not an issue.
I think societies or subjects or sports hire out rooms there to hold certain conferences/talks/events but obviously if you're never a part of these particular societies you don't need to go there or be involved. I think in 2nd and 3rd years, more people get involved in student council and stuff, but not everyone does.
I think, from what I've gathered, other universities hold fun events and have a really good, cheap bar that makes their union good. Bristol doesn't, but it's just not a problem. Bristol nightlife is so diverse - there is quite literally hundreds of clubs/pubs/bars combined. So we don't need the union. I can imagine a smaller city university or a university with no big town/city attached to it (like Warwick or Durham) would use the SU bar/club a lot more and have a lot more events there.
Most people don't even know what the union really does (apart from the work it does with the student council), and we don't really need it anyway. It's also a 15 minute walk from the uni campus (which also has the libraries and gym centre in it).
What I'm trying to say, essentially, is don't base your decision on the union. It's not bad, it's just not as much of a hub in such a big town/city like Bristol.
Also, why are you even concerned about the union? Never ever heard of someone considering that when applying to university....