Reasons to not use playmode unit tests?

Playmode unit tests seem really awesome - I can do things like, make sure spell cool-down timers work properly.

What are people's experiences with playmode testing? It warns you that your build may get bigger and slower, but has this been an issue for anyone? I'm targeting desktop, so 10-20 MB of additional payload isn't a huge deal. Is it even that bad?

This also seems like an issue that can be fixed in the long run - no reason a production build should be affected by testing infrastructure.