Don McCullin, one of the most famous British (oops he prefers English) photographers, made his name through war photography. He said that when he began you might get five photographers covering something in the same place. When he finished you could have five hundred turning up. Maybe it's because I'm a Londoner - that I moved to Nice.
But of course, 100% unionisation of any sector would be beyond my wildest dreams.
Those reasons are why photography is likely to remain a hobby (at most paying for the kit I have, over a number of years). Ad astra per aspera
It did strike me when I read the website that they are fairly powerless to do a great deal for photographers who work freelance and although they give recommendations on rates, they are fairly worthless if a large number of amateurs/enthusiasts sell their work for next to nothing and are ignorant about protecting their rights - just because it is nothing more than a hobby for them. Ad astra per aspera
Unionisation of freelancers is always very, very difficult to arrange. I'm sure a lot of photographers don't even realise the NUJ is relevant to them. And freelancers by nature are solitary. It's not like we'll be standing in picket lines, yelling 'scab' at people who are taking phone calls from editors.
I'm not sure what the solution is. (Actually I have an idea, but I'm still wondering how to implement it.)
But it is an issue as to how unions can support freelancers, for all the reasons you mention and more. Ad astra per aspera