Ahriman
Tyrant
The main reason for multiple unitcombats is for promotions. The promotions are made available by unitcombat and they generally give advantages against unitcombats. If we merge all three categories into one, then we can't give just thopters, for example, a bonus against guardsmen. So I recommend keeping three unitcombats, unless you agree that all three will have identical strengths/weaknesses against other unitcombats.
I agree that hornets and thopters should have different access to promotions. Not because of strengths/weakenesses, but because aircraft should be getting things like improved interception and evasion and collateral damage, while thopters should not.
I'd consider merging thopters and carryalls though, but the only real reason for this is to make things slightly cleaner.
The downside of separate categories is that the rocket trooper type units then need a larger entry; they need to get a bonus against 3 types of units (vehicles, hornets, thopters) rather than two (vehicles, aircraft). But this is minor.