1.Chrome 70 on Linux doesn't drop frames for 1080p but 4K is unwatchable.
2.Windows 10 on same PC is better for 4K but drops some frames for 1080p as well.
3.Safari doesn't have VP9 codec; so there's no support for anything beyond 1080p.
So Youtube on Xbox is the best experience I've had w.r.t to quality of streaming. But it zooms by default on my ultrawide PC monitor.