I'm pretty certain that this is an issue specific to your configuration. I test forum e-mailing pretty frequently on fresh installs (at least once per week) and haven't seen any issues.
The general culprits tend to be:
- forum is hidden from users
- users are not enrolled in the course
- users are not subscribed to for the forum and/or post
- users have disabled forum notifications
- you have not waited until the post timeout has expired (default 30 minutes unless you modify it, or click the "Mail now" checkbox)
Can you please confirm that you have checked each of these?