I got a car whose cruise control state was never displayed (the mechanic used the wrong dashboard), but since it worked anyway, I accepted it. So that's a data point (me) that modal buttons do work.

But "should not depend in the GUI to keep up" is wrong, I think. It needs at least one LED per mode. Because sometimes it's a bit annoying. Rarely cruise control doesn't engage or turns off by itself or something caused the cruise control mode diverge from what I have in mind, and a quick glance would help to resync.