In your games, do you allow flying combatants to be safe from grounded opponents (excepting, of course, attacks expliciting defined as ranged attacks like blasts or thrown cars)?
To put it another way, can your close combat fighters still punch your flyers? If so, do you at least make them describe how they do it?
Or do you just say, "Flight has no inherent defensive benefit unless an Ability says so?"
I'd say that it's a narrative thing. If your flying thing is airborne, then it's probably safe from certain things.
However, that doesn't mean that heroes should be locked out of interaction. Attacks can be emotional as well, or someone could take a Risky Action to hit a foe by doing something well, risky. Or, maybe a person uses a Hinder or Overcome to bring a flying character back to earth. Very much though, I see it as a narrative effect.
Also, throwing things at your opponent is a staple of superheroic action.
For my portal generator, he has a saying: "I learned the hard way that some folks can fly. Doesn't make them any better at dodging falling cars, though." ;-)
It's on the players to come up with a way to deal with situations like that. If I'm running the game, I'm not granting any specific immunities, but I'm also expecting the players to come up with something narratively that will address the situation.
If flying is all they can really do, no blasting or guns or anything, then they're going to be coming in to punch things themselves unless they're pure support. That leaves them vulnerable to counterattacks!
It really depends on how you decide to do things.
Given the structure of the dice pools one makes (Power + Quality + Status), if one has the right powers, getting up close and personal with a flyer isn't that hard. Leaping is a great way to reach a flying target (it works for the Hulk at least), but Agility and even Speed have been used by various heroes and villains in comics to launch themselves into the air. Using the Acrobatics quality with a more offensive power could also work.
And that isn't the end of the possibilities. Have a super strong character? Have them Boost your close combat character by throwing them at an airborne opponent... "Fastball Special" style!.
In short, the flexibility of the game design makes it easy to justify how almost any character can attack any opponent. All one really needs is a little creativity.
its really hard for me to think of a situation in which merely being airborne would make it impossible for a player to affect the target in some fashion.
I could see them being unaffected by certain actions, like if the player has an ability that makes the ground a swirling whirlpool of sand for a large group hinder that might not work on a group of flying enemies.
I could see the flying enemies having an advantage. You decide to give them a plus 1 bonus on minion saves against attacks coming at them from the ground because they are really good and mobile flyers.
But i can't think of a situation in which the character wouldn 't be able to make some kind of attack.