The biggest factor against them is that Suntour rear derailleurs are not at all compatible with Simplex dropout mounts. The PX-10 came with Suntour when I bought it second-hand, and it made wheel changes a challenge. I might try some sort of quick-release skewer...
I have a Velosolex with Simplex dropouts. I was able to cut a notch in the dropout with a Dremel that will catch the B screw from a Suntour V-GT just fine. The axle to pivot distance seems long, so the B screw is backed off pretty far, but it works well, much better than the Simplex derailleur that was originally spec'ed. I have used it with up to 32 tooth, seems like it would work with a bigger cog, currently running 14-28. By the way, this was done in 1976 when I bought it new.