Eye bones work best if placed vertically. The VRChat Avatar Descriptor struggles to handle using horizontal bones, vertical placement makes setting rotation values for the eyelook settings MUCH easier.

When rigging the eyes on a character that has nearly flat eye sockets , you typically need to place the eye bones incredibly far back, and sometimes even on the wrong side. This takes advantage of a very large rotation circumference allowing the eyes to mostly rotate flat.

This can sometimes cause over-rotation in MMD Worlds and general slightly strange eye movement in-game though (nearly unnoticeable however)