comparing to Grbl, or FluidNC

but in theory, a Grbl component could completely replace the EMCMot+Stepgen components in LinuxCNC for stepper motors signal generation.


Hmmmm ok

the answer is simple,
GRBL,... is a firmware for CNC milling machines, laser cutters and the like.

LinuxCNC is a highly configurable motion control platform,
it can synchronize all kinds of input and output devices,
also from external not self-controlled movements.

It can handle all kinds of actuators, not only step/dir or RC servos. It can handle all kinds of protocols like Ethercat, CAN, Modbus, .... and even mixed.

It can handle
all kinds of inputs, glass scales, encoders, camera images, .....

It also doesn't need gCode as input, that's just another module in the chain


So if you want to operate a simple hobby CNC, you are well served with GRBL and so on, for everything else LinuxCNC is the best choice.

One aspect that I did not see mentioned is the time delay between inputs and action on some of the systems.
My Marlin based 3d printer takes ages to accept a feed override or pause instruction. (seems to finish the complete "look ahead", before it stops?)
Something that I would not be happy with. For a milling machine things quickly become expensive and messy when the machine is able to destroy itself.