While my intuition would be that incidents are fairly rare for social reasons, intimidation/coercion with postal ballots is essentially invisible, until and unless people come forward and complain (which intimidation and coercion itself can be deployed against.) So, even if it was common, the elections would look smooth by the things we usually look at.
It's also a precaution against future regressions: you want to design protections like this for the worst political climates, not the best, so you'll be ready if e.g. an employer starts thinking they can get away with abuse during a recession or a union leader decides desparate times warrant desparate measures.
It's much easier to prevent that kind of thing rather than repair the damage after the fact, similar to the problems we've seen with gerrymandering.