I have used the htdemucs_6s a bunch, but I prefer the 4 stem model. The dedicated guitar and piano stems are usually full of really bad artifacts in the 6s model. It's still useful if you want to use it to transcribe the part to sheet music however. Just not useful to me in music production or as a backing track.
My primary use is for creating backing tracks I can play piano / keyboard along with (just for fun in my home). Most of the time I'll just use the 4s model and will keep drums, bass and vocals.
Yeah I could see that. We had better luck with the 6-stem, maybe it's because we had both rhythm and lead guitar in the mixes, but the 4-stem version didn't work as well for us.
It probably also depends on the channel separation for the individual instruments in the final mix and any effects applied. A stereo chorus effect on one of the instruments can really interfere with the separation from other instruments from what I can tell.
Piano (or various keys), organ and some guitars (with effects) have a lot of frequency overlap. The model struggles there.
My primary use is for creating backing tracks I can play piano / keyboard along with (just for fun in my home). Most of the time I'll just use the 4s model and will keep drums, bass and vocals.