For a union vote that happens once and has millions of dollars on the line for thousands of voters? Not an insurmountable cost. Also, again, because mail-in voting happens over weeks, you don't need that many goons, just have the same goon visit everybody once in that period of time, when everybody understands they'll be punished for voting without them there.

In ordinary elections you get something different -- decentralized coercion. Your boss says hey, I want everybody to vote, everybody bring your ballots to work on Tuesday and we'll all fill them out together. Now they don't even have to look at them, all they have to do is put a fear in people that they might so that people fill out their ballots the way the boss wants in case that happens. The same thing happens with many bosses in many places, so the total effect is significant even if they're not coordinating. People fear for their jobs if they report it.