If your boss, union leader, controlling spouse, etc. tells you to vote a certain way, they would gain the ability to demand that you give them your identifier if it's public or, if not, login and show them the final vote status. This is very hard to detect and anyone proposing a blockchain needs to learn why election systems are designed the way they are first since this is not a hypothetical problem.
We have plenty of jurisdictions with vote-by-mail who've been doing it for a long time. Long enough that we should have some idea how common this occurrence is. It seems like the answer is that one vote isn't worth nearly enough to risk the penalties involved.