Of course, even your example units don't always behave like you want. For velocities, 150 megameters/second + 150 megameters/second ≈ 240 megameters/second.
Sorry, can you explain why the velocities should add that way? I've always just seen velocities add like regular old vectors, so this looks very strange.
But when some unit looks like a meter or gram (especially using the micro- prefix here), it should behave accordingly.