pdksh (at least the one in Debian) has numerous bugs; try the shelltest.od script in redo to see some of them.
I agree that "just see if it works" isn't a good portability strategy if you only test one shell. But the idea is to get as many shells as possible and try them all, or (as redo does) to write a script that accepts only shells with all the expected features. Blindly assuming POSIX compliance is a dangerous way to go, since there's no guarantee that it's in a shell just because it's in the spec.
I'll grant you that using a feature that's both in POSIX and in all the shells you test is the best option, though :) $() is in both categories.
I agree that "just see if it works" isn't a good portability strategy if you only test one shell. But the idea is to get as many shells as possible and try them all, or (as redo does) to write a script that accepts only shells with all the expected features. Blindly assuming POSIX compliance is a dangerous way to go, since there's no guarantee that it's in a shell just because it's in the spec.
I'll grant you that using a feature that's both in POSIX and in all the shells you test is the best option, though :) $() is in both categories.