Um, I use grbl on my laser engraver. Had no problems.

The only machine I have with marlin on is the i3. And it is hands down the most unfriendly firmware I've ever come across.
Doesn't even have a load filament option.
So if there's a viable alternative to marlin - I will always go for that :-)

If there were any way to ut something else on the i3 - I would in a heartbeat.
Repetier is great firmware, as is sailfish (what the replicator clones use).
But marlin - just is horrible at every level.

PLus lasergrbl will update the grbl firmware direct from itself.
marlin you have to bugger about with ide - horrible software.

I have yet to see any benefit of any kind to pissing about with marlin.