My Elgato Cam Link 4K has captured everything I’ve thrown at it. It’s probably worth mentioning that I use a Micomsoft Framemeister XRGB-Mini to clean up the HDMI signal. The Cam Link did not work well from the OSSC directly. Some notes:
Audio rarely makes it to the Framemeister over HDMI so tap your analog audio and feed it to the D port. There’s a menu option for HDMI 1 to pull audio from here.
Try turning on OBS “Use Buffering” and turn off Framemeister “Sync Mode”. This gave me the least amount of judder but 100ms of lag. You may need to tinker here depending on the timing of your hardware.
You probably don’t want to be playing with 100ms of lag. If you’re using an analog monitor, split your analog signal as usual. For HDMI, use a splitter after the OSSC.
Having two scalers is very powerful (and expensive). You can, for example, play at 1280×960 and capture at 640×480 or even 720p. Don’t forget to change sharpening to from 0 to 1 if things get blurry while scaling on a non-integer.