Compatible in what sense?
As in...I have a friend who's building a custom library site through Perlbrew and some other coding stuff, and we use SMF for the forum portion of the site. I think what he wants to be able to do is have the tickets work for both the library he's building AND SMF...so if something happens to a user on SMF, like they get some weird error, they can report it through the support ticket system, and then he'll get notified of that error.
Why not use SimpleDesk that does the same job but was designed expressly for SMF?
I hadn't heard of/found SimpleDesk yet, but my answer would be the same as above, I think. I think what my friend is looking for is something that will work for both the library he's building and the forum at the same time.
For what it's worth, osTicket is already built in to our hosting through cPanel, and it's free regardless, so it's not like it's going to be an inconvenience for us to use osTicket in that sense...but if we can use it for the forum, too, that would be helpful.
For the heck of it, I'll at least look at SimpleDesk, too.