>Feels like it's a very high false positive rate and one that pushes all the time costs onto the applicant (multiply ~2hr by say 5 companies that want you to do a coding challenge).

It is, I wouldn't entertain companies that do this unless you really need a position.

This is the trifecta of bad interviewing tests:

- No compensation

- Stupidly restrictive time limit

- High chance of not getting feedback

Personally I would turn them down and then explain these three things. There are plenty of companies with less annoying hoops.