The OSSC does not do any signal normalization; whatever refresh rate and sync cadence it receives, it passes on to the display, whether it’s to spec or not. For example, the SNES’s 60.08Hz refresh rate and jittery sync are passed directly to the display. (Although I’m not sure how the refresh rate is affected by line2x/3x/etc.) You can fix the jittery sync on the NES and SNES with a mod to increase compatibility, but if your display can’t take the non-exact refresh rates or [effectively] PC resolutions, the OSSC won’t help you.
In my case, my older Samsung LCD just doesn’t like Line3x/4x/5x, so I’m stuck with Line2x and relying on the TV’s scaler.
The Framemeister, on the other hand, can do signal normalization (and format conversion, like PAL->NTSC) so you get standard 60Hz 1080p (or whichever resolution) output, which any HD/4K TV should take without issue, but this is at the cost of 1 frame of lag and the additional cost over the price of the OSSC.