This is mostly irrelevant with modern compositors. If you force the compositor pipeline, the worst it will do is drop frames. To my knowledge, the only hardware that doesn't do this by default is Nvidia GPUs, which have a specific configuration option to enable it.
This is not true. It's caused by drawing out sync with the vblank cycle. It has nothing do with specific hardware. Older versions of X11 didn't even allow drawing to be synced to vblank (the start of the raster retrace).