ANGLE_instanced_arrays is the required WebGL extension, but I think it’s pretty universally supported at this point. I would expect a hard failure if it’s not supported, rather than just degraded performance.
I don’t think InstancedMesh supports front-to-back sorting, like separate objects would have? Without that, it would be pretty easy to stumble into situations with bad overdraw using instancing, so be careful of that.
Also see: Merge or Use instance and Native render make no difference with Chrome plus Integrated Graphics Card