I'm not expert at that but from what I know, main difference is that OpenGL ES has fixed pipeline rendering (that GLB is using) and in ES 2.0 everything is programmable by shaders, so even for rendering simple 3d cube you need to use vertex and pixel shader fragment programs, this would either require rewriting part of GLB engine or add some translation layer or default shader code to at least have 2d rendering working (this would be simpler as basic shaders are enough), many 3d GLB functions such as lights/shadows/specialfx would require more work.
The lack of shaders on Android in GLB (due to ES 1.1) may be annoying as they are handy for some effects like realtime 2d mirrors, drawing filters that emulate older devices/consoles, and other - like using one base sprite with colorkeys that are switched to draw units for different players or whatever.
