Screen tearing, bad performance, bad touchscreen support (not independent of mouse cursor), and especially important these days: no real HiDPI support (practically impossible to support different scales for different monitors). It's not good enough.
Screen tearing: both Intel and AMD have hardware-backed "TearFree" buffering to prevent tearing. Bad performance: citation required - in my experience, Xorg is way faster. Touchscreen support - in most cases just works out of the box thanks to libinput. No HiDPI support for different scales per monitor - simply wrong - this is trivial with xrandr.
I have a machine with a supported Radeon card (open-source driver), another machine with a supported nvidia card (binary blob driver), and another two machines using different intel onboard graphics chips (open-source driver)
the radeon and the intel (both drivers which work with it) have issues putting out a stable jitterless 60fps without tearing (with tearfree on, and various combinations of with/without compositing/glx/...)
I'm using the intel driver as well, and it's definitely not perfect. But it's pretty close, at least for me - I get hardware-accelerated video decoding with VAAPI, no tearing, and excellent input latency (~3ms).
sadly I can see the jitter/tearing, vs. on Windows where it's perfect
(with 4k or higher frame rates it's far more obvious)
Sure you can. Just pop the widow into floating mode; I think the default is meta-space.