Doesn't this always force delays? If I have CMD + Right Arrow assigned to some other function, the system will have to wait to make sure the Up Arrow isn't coming soon before dispatching the Right Arrow function. Of course we want the delay short, but that makes the multi-key shortcut less reliable.

I haven't tried it, but the tradeoffs don't sound great to me. Only allowing multiple modifier keys avoids all of these timing issues. You seem to be comfortable with it, but I think most users would trigger a lot of spurious inputs.