It's "Report Bad Post" rather than "Report Bad Thread."

If you think the whole thread (rather than just one post) is bad, you could still click on the "Report Bad Post" icon and explain in your message that you think the whole thread is problematic.

As for e-mail vs. PM, remember that there is a limit of 100 saved PMs in each person's maibox, so although the "confirm receipt" option is nice, you may be filling up someone's PM mailbox (or even find it is already full). For this reason I often prefer e-mail over PM.