Right, I was only talking down about grbl a little bit

, it's pretty remarkable for still supporting 328p parts, and a lot of the Marlin magic
is 3D printing specific. I was more remarking that the degree of separation between the additive and subtractive desktop manufacturing communities surprises me, and thinking "more sophisticated" in terms of supported kinematic system types, the screens and SD support you mentioned, and g-code dialect. It is certainly the case that they are pretty well filling out an ATmega2560 with 3-4x as many pins/timers/PWM channels/etc. to do so.
Feed override is definitely a fussy feature since it means the machine isn't actually directly doing what the gcode says, and in doing so interferes with look-ahead/buffering etc. I also know my LinuxCNC workflow tends to involve twiddling overrides to adjust out misbehavior, and I've been using overrides pretty heavily since Pronterface sprung a good override control for dealing with iffy bed adhesion or features (and also that I've had it do weird buggy things on subsequent prints when the override was in use, though to be fair that machine is running a rather ancient customized Marlin build).
I'm hoping for good things in terms of both features and re-unification to happen as the LCP17xx-based designs get dominant (which I'm pretty sure will happen), as much as I love working with AVRs and especially Arduinos, it really looks like the more capable devices will ease some of the inherent compromises of using serial-attached motion controllers.