If humans are the only allowable playable race, maybe only in rare occasions do you have an elf, Then humans can have the AMF field naturally. It doesn't unbalance the game because every group would have it, and instead the game is designed around the face that human mercenaries are assaulting strongpoints where all but the strongest magic is shut down by their presense.
If humans aren't the only choice, but a PC mercenary company might include a human commander, a human sharpshooter, an elf negotiator, a bug magic warrior, and a squid wizard. Then I don't think humans having AMF's really works. Natural resistance? maybe, but unless you need 100 of us in any one place or more to cause the field you'd be screwing over your team-mates. This part however gives the merc company magic to defend against magic with, and tech to fight magic with. Plus guns are all well and good but sometimes a fireball is what's necessary.
Maybe part of being human is every member of your party has a +1 resistance to magic, simply because you're around. If you got an all human party, it's +5! But you can still choose to take a bug alien, get the benefit of being around humans who somewhat negate magic, and still cast your spells.
In fact, it could be that humans started having to only go around in smaller groups. 5 humans have got some resistance, but if you try to assault an enemy stronghold with 100 humans, none of us and no one around us can be affected by healing, magical shielding, haste, and the like. so when the enemies teleport lava above our heads, we're screwed since lava isn't magic, moving it above our heads is.
Until we can properly combine tech and magic in a single soldier, we need tech and magic in a single unit. To much tech and it's weak to magic, too much magic and it's weak to tech. So it means merc groups of about 4-5 people, some humans and some aliens, would be perfect.