The problem is not just that it is flickery, it's actually often unusable.
There are so many sites that use on-hover to trigger a menu (often with sub-menus), so that if you misplace your pointer even slightly, the top-level menu option you selected disappears.
I've lost count of the number of times I have to play the game of selecting from the top level menu, back down to the item I actually want.
The problem is not just that it is flickery, it's actually often unusable.
There are so many sites that use on-hover to trigger a menu (often with sub-menus), so that if you misplace your pointer even slightly, the top-level menu option you selected disappears.
I've lost count of the number of times I have to play the game of selecting from the top level menu, back down to the item I actually want.