This depends almost entirely on intelligence and tactics. If the dragons are dumb (animal intellect), they're useless to more advanced humans (although the ability to take off at will is nice, they're not durable or all that fast, still need fuel, and can't carry as much as planes, etc.) If the dragons have human-level intelligence, they’re far less useless, but still not all too useful due to their lack of useful limbs, and due to the fact drones outclass them. (I'll be assuming dragons are animalistic.) However, if their allies have intellect, dragons can become a major force in combat, although not that much. How, you ask? The answer is tactics (and armament). Take this example:
That’s a dragon what are you talking about
If the dragons have artillery (missiles?) mounted to their backs, they will be much more useful. If they just hover/circle above a fight and drop bombs or even large rocks over the enemy position, far out of the range of small arms fire, they will be much more useful. They can also serve as cheap transport/supply droppers that can be stationed close to the battlefield to take advantage of their ability to pretty much be ready at a moment’s notice (compared to planes at least). 160
They also don’t really need maintenance from what you’ve told me; they can get their own “fuel” too.
As I said, if dragons have machine guns mounted to their backs, they’re not as useless. Their actually quite small size and good enough stealth may make them good at ambushes/hit and run, perhaps with machine guns mounted to their backs, or maybe poisoned claws, and their resistance to small arms fire and favored close quarters combat weapons like knives (if they can withstand bullets they can probably withstand knives). Don’t forget Kevlar exists also. Strap 2 of those hand-held rocket launchers to their sides and adapt them to allow the dragon to fire them, and you have effective air support that can zoom in on unsuspecting soldiers at 50 mph and be gone before any return fire occurs (night ambushes with dagonightvision goggles would be viable).
Finally, remember this: Humans are just as (more) vulnerable to bullets. Even if dragons are larger, they can fly and provide air support out of range of counterattacks (because gravity).
TL;DR: Dragons are, at stock configuration, weak. They do have a niche due to their ability to be ready really fast (unlike planes) and not being incredibly loud (unlike helicopters), and their lack of maintenance, but they’re pretty much infinitely slower than planes, have pretty much no carrying capacity, and have garbage durability if they can be effectively made completely useless by one lucky shot with a revolver (whereas a plane has somewhat decent armor).
P.S.: Dragons too weak? Buff them. You’re the writer. Make up something (maybe the dragons evolved to have a ton of muscles in their wings to allow for stronger armor?). 160