Sending troops down to a planet is just giving the locals some targets to fire at. This will empower the resistance, even if their kill ratio is something like 1:20,000. But sending down a cloud of germs and then withholding the antidote will strip the locals of all power... Data had it right in "Ensigns of Command": resistance is futile.
OTOH, firing at stun from orbit is likely to kill the elderly, the young and the cute puppies and kittens before it will start to have any impact on well-protected fighting forces. "A Piece of the Action" was quite an anomaly in that said fighting forces were all out in the open, wearing felt hats for protection, with the molls and babies hunkering in stone buildings!
As for Nimbus III, the opponents were wielding sharpened avocados and pebble guns - hardly a case either for or against planetary assault. Were the roles reversed and our heroes had the pebble gatlings while the baddies had the pebble-proof riot shields and phasers, score one for the baddies.
In short, we never saw infantry warfare in Trek. But we did hear of shipments of troops, so supposedly some sort of manpower-intensive fighting was going on nevertheless.
Clearly there's none in DSC, though. Burnham got about 8,000 people killed in the first day of the war. Half a year later, when Harry Mudd comes to pay a visit, the total count of fallen is 10,000...
TImo Saloniemi