Huh, added to the long list of things I never knew about Keyboard Maestro :). I thought I'd replace the AppleScript in my Macro Usage Counter macro with the URL call.
However, after testing, the AppleScript method is much faster, at least on my iMac. It takes about .2 seconds to open a macro via UUID with AppleScript, but anywhere from .4 to .5 seconds to open it via the URL scheme—I'm guessing because the OS gets involved to check for the registered handler for the URL?
So for now, I'll stick to AppleScript—it's not a huge difference, but I'll almost always pick the faster solution, even if it's a bit less elegant.