What you said makes sense. But I feel like representing lots of mostly-unrelated industries is a weakness, too?
Like, imagine you're a teacher and you're on strike because of conditions at some steel mill you've never heard of. You want to support your fellow workers... but also, you're forfeiting your income for a cause that's not yours, for people you don't know.
Typically, we’ll run unions have “strike funds” which are pools of money they distribute for strikes.
Additionally, I think backpay is sometimes part of a deal.