No, it is not limited to 60hz, it does 50hz just as well. The majority of the Youtube posts should be in 50 fps. The recording software takes the incoming frame rate and saves that way to an MP4, and if the frequency changes, like 60 to 50hz, a new file is created on the spot. I can not speak for the mono ST screen, but that problem might be more OSSC related.
For example. in this posted video, you can see the change happen, although I use the video production software to output a video that is totally 50 frames per second - https://www.youtube.com/watch?v=ItxYCZ7Uc9A
Go to about 0:51 and you will notice the screen changing, and that is the hertz change from my 1040 STf, 60 to 50..
It is kind of cheesy, but I leave the hertz change from my US ST in all the videos, but encode to the most used framerate. So, if something runs at 60 hz, I encode and post at 60 frames per second, and if 50, I do the same. It is the real experience, at least for us in the US. (old man voice) I can remember messing with my Goldstar SC1224 to make the scrolling stop early on with 50hz software.
One thing to note is that there is some occasional frame dropping, not every time, but sometimes. That may be caused by the HDMI splitter I am using, since the Startech device does not have a HDMI pass thru. I can't say I have ever really noticed it causing a problem in a finished video, but I think if something was paused in the exact from, it might be caught.
I use Shotcut for assembling video (I could not justify paying Adobe for Premiere) and actually set the output frame rate as the last step right before encoding.
Hopefully something of use was in the above, and let me know if you have more questions!