Playing 2K on a 4K monitor: Any setbacks?

I'm intending to buy a 4K (27 to 32 inches) monitor for photo editing. I also play games sometimes, but my rig isn't powerful enough to run 4K games smoothly.

So I just thought, I'd put the game resolution to 2K and play away.

Other than the obvious sharpness reduction, do I need to be aware of any limitations or risks (i.e. GSync or FreeSync not working well, performance reduction due to rescaling, monitor response reduction due to not running at native resolution etc.), or will it depend on the specific monitor?

Thanks in advance.