In my experience it's not risky at all, I have a cheap 1080p acer monitor that I have overclocked from 60hz to 74hz and have used it like that for years as a second monitor without issue. Nor have I ever heard of someone damaging a monitor by overclocking it. However I believe it does increase heat output marginally so it could possibly decrease the lifespan of the monitor, how much if at all will likely depend on the specific monitor.
Overclocking a monitor can screw with gamma levels on some panels, so if your picture gets brighter or darker after overclocking that's why.