i run stereo, the 4 other guys run mono for simplicity. i also mix and lead the band, so stereo is nice for me.
i'm not sure i can answer your question exactly correctly, but i'll try.
the main signal path of effects are definitely true stereo. a hard panned left mono signal into the stereo fx engine will only have reverb appear on the left speaker.
however, when you pan the IEM Bus sends (x32 calls them Buses instead of the usual Aux for monitor sends), those pans aren't sent to the FX engine - there's no way any FX engine can have 6 stereo inputs with different panning (1 main, 5 band members) without having 6 dedicated outputs.
so i think the answer is no, IEM effects returns aren't true stereo unless you somehow route a dedicated fx instance . the sources and channels are definitely true stereo once you pan them. but without dedicated in/out to your IEM mix, there's no way for any FX engine, internal or external, to have your own dedicated panning, vs other's panning at the same time.
that said, usually during a gig i'm way too focused on playing and other things to discern if the reverb on a panned left vocal is only coming out of the left side. so i just feed the FX return back to my Bus send for some "air" and it sounds amazing.