I don’t think that it’s necessarily the OSSC at fault. Usually the problem is that older consoles tend to output non-standard video signals that CRTs simply don’t have a problem with, but contemporary digital displays do; and the OSSC isn’t designed, and therefore lacks the necessary hardware, to normalize these signals in the way that a Framemeister does.
One of the problems I’m aware of with the SNES, and the NES as well, is that there’s some jitter in the sync signal. Someone more knowledgeable can correct me, but I believe the gist is that every other frame gets cut a hair short, which leads to an inconsistent sync. Digital displays expect a consistent sync cadence, so, when they don’t get that, they’ll drop sync, or simply not display the image.
There is a dejitter mod in the works (at the link above), which might help your compatibility issue with your SNES and TV.