I haven't specifically tested any of the Steam Store add-ons yet, maybe someone who has can chime in.
To be more general, certain 3rd party aircraft work flawlessly, others have occasional bugs, and some refuse to work at all
I've heard of some people having a great time with PMDG airliners, others having no luck. I'll be working on support for some of the major aircraft though, like PMDG and A2A. I'll also be fixing others as they are requested and time permits. My hope is that as I improve aircraft support they'll all start working, but you know some airplane is going to do something a little bit extra special that causes conflicts