The search in bbPress 2.0 is purposely limited. It’s possible to remove the restriction with code (a small plugin) but I wouldn’t recommend it, since the actual number of users that search your forums is surprisingly small.
For an example, had you searched these forums, you would have found several topics about this already. Instead you naturally skimmed the site, didn’t see an immediate answer anywhere, and asked your question.
Does this little bit of code in the functions.php work? Is this the plugin you are talking about?
https://bbpress.trac.wordpress.org/ticket/1575
I’ve actually tried using the search bar here several times but it wasn’t working for a while, it was just bring up a broken page (I don’t think it was a 404). But it seems to be working now.
Is there any way to create two separate search bars? Or drop down options like in Buddypress to search Members, Groups, Forums, Posts?