[huhn]

There are people who like to be on the SAFEST side. "DO NOT DRY YOUR CAT OR DOG IN THE MICROWAVE OVEN"
I am an electric engineer and know some of the electronics stuff. So: Your output device can do some crazy optical things or freeze/hang, but after repowering (power off - power on) everything is fine again.
Bytheway overclocking of computer components by itself doesn´t do the harm (I speak of processor, GPU, memory). If you clock too high it errors. Nothing more. The danger comes from too high voltage or too much heat. If you simple rise the clock you very soon come to a point where you have instability. So people rise voltage so rises the heat so the components are grilled. The overCLOCKING part is totally secure (if temperature is within allowed limits).
And thats the same logic with our custom resolutions. No voltage, no heat. And in this case even no OVERclocking, but some kind of exacter clocking for our wishes.
A damage warning in this context exists for the people who expect such a warning placed

when a screen freezes, which should be very rare anyway, you can easily lose your settings or with other words your paid calibration.
because a device will not catch fire doing this doesn't mean there couldn't be other major problem.

the biggest problem of overclocking isn't killing the hardware it is system stability.

tweaking 23p or 24p has is a very low risk tweak if there is even any real risk but creating a 47/48 HZ refresh rate is something totally different. of cause just an example.

your screen could drop/repeat frames.
your screen could start blinking.
accidentally overriding the EDID that you didn't get any picture at all anymore.
your device could freeze.
and a lot more. just look at the CRU notes and quick start.