Do you know what puzzled me from a military point of view in this film and in the first one? The total absence of drones. I mean, they are a major part of the conflict in Ukraine NOW, let alone what they might become centuries from now. I guess the reason from a narrative point of view is to make the conflict between the protagonists and the antagonists more dramatic and personal, but it doesn't make much sense. I wish they had said something about it, like "Ah unfortunately the planet's radiation makes it impossible to use drones" but never mind!
In a dying world full of twenty billion people fighting over dwindling resources, heavy automation means fewer jobs available, means more people getting sick, starving, turning to crime in desperation, perhaps even revolting against and sabotaging the companies that eliminated their jobs in favor of automation.
Now along comes the RDA offering settlement on a new world, a steady income, the possibility of improving your lot in life, and the need for lots of settlers, growing with each sortie, to establish a beachhead on Pandora and begin to relieve the population stress on Earth. They don't want drones, they want jobs for as many people as they can get, with the caveat that they are as expendable as drones are to the company. I mean, in two decades, they didn't even increase the thickness of the cockpit plexiglass to prevent a Na'Vi arrow from going through it, which seems pretty basic.
So, the reason for the lack of drones:
-- The RDA needs to make work for as many people as possible to increase settlement numbers and give people incentive to take a risky trip to another world.
-- There may be existing terrestrial resentment to automation that fully replaces rather than simply supplements a human, which would explain the AMP suits and exoskeleton the general was wearing being commonplace but drones and robots not.
-- Given that this is Cameron, he may have his own worldbuilding for future Earth that we're not privy too yet where AI drones got out of control during all the human resource wars and turned on their creators for a bit, making them untrustworthy in the eyes of many, and especially the former soldiers working private security for RDA.
-- The aforementioned magnetic fields and "flux vortex" issue which wreak havoc with electronic systems but don't affect humans would render drones useless and cause them to crash in many of the sacred regions favored by the Na'Vi.
-- The RDA does seem to be somewhat restrained (I use the term very loosely after that nigh apocalyptic landing sequence) either out of concern for poor PR or legal restrictions placed on them by the world governments, or else you'd think they'd just fuel-air explosive bomb every Na'Vi settlement they find, or release some Na'Vi-targeting genetic virus to wipe them all out. Clearly, they have a long leash for doling out violence, but total extermination of the indigenous population seems to not be permitted. So wiping out a bunch of native tribes armed with bows and arrows with unmanned drones might cross the line for the folks back home, even though they're turning a blind eye to all the other slaughter.
-- From an out-of-universe standpoint, there's no dramatic tension if it's just drones fighting. Sure, you don't want the Na'vi to die, but you also have Trudi in her chopper, and if it's just drones, of course you're going to root against them. But if they're your fellow human beings, and you're actively rooting for them to lose against an alien force, that's a unique and interesting turnabout.