The hierarchies, formal and informal, mean that some get away with not working very much at all, at least not for their employer, while others are exploited. I don’t think unions can do much about this - but I may be wrong.
I think this is spot on @ExUCU. The issue is systematic - but perhaps in a different, and more complex ways, that UCU says it's systematic.
A crucial issue, that has been discussed in these boards over the years, is that the type of work we do does not lend itself easily to collective action. In some ways we are more akin to freelancers (like a performer, a consultant... - as also evidenced in the working hours flexibility discussed above) than employees. Except for teaching, with any other activity that we cease during industrial action (research, networking, external partnerships, etc.), we would have to sustain significant damage to our own profiles before this starts to harm the employer in any way.
I started as HoD this academic year, after 10 years of seeing a chronic staff shortage in my department that meant that even my internally and externally funded research periods could not be protected as they should. I am now starting to see that a lot of the overwork is: a) of our own making, given that the university insists that we teach fewer courses, but we have insisted on teaching more (this ties in with the above: apparently our "identity" as a department and as individual researchers is tied in with differentiating ourselves from others by offering a much vaster course choice); b) very unequally distributed, with successive HoDs allowing some staff to not do much while piling up the work on others.