Flyers are a big deal in 6th Edition, obviously; they added what was essentially an entirely new type of target to the game. At the same time, however, it’s very easy to forget about their disadvantages: they never affect the board turn one (and often won’t on turn two), they have much more limited fire arcs and movement than any other unit type, they generally cannot contest or claim objectives, etc. All-flyer armies are, for some or all of these reasons, usually pretty terrible, just as any one-trick pony army usually will be.
However, that’s not to say that flyers are not dangerous. Quite the contrary, in fact: flyers represent a major defining factor for this edition, one that has greatly changed how the game is played and how armies are written. That might seem like a contradiction with the above, but it’s not, because there are essentially two types of flyers in the game: good flyers and bad flyers. Good flyers change how you have to play; bad flyers can be ignored.
That is, of course, a bit of hyperbole to prove a point, but the essential idea stands: there are some flyers out there that will make you change your entire strategy, and there are some out there that will not. When we talk about “lists being weak to flyers” or “needing a way to deal with flyers,” we are in all cases talking about the former kind, not the latter. Moreover, it’s not simply points efficiency that defines whether a flyer is “good” or not in this sense- it is the role that flyer fills in an army.
What is a good flyer?
It’s a pretty simple question with a pretty simple answer: good flyers let you take the enemy’s objectives from them. Remember, 6E is a game of objectives; five of the six missions use them, and in most cases you will have to get across the board to get anything beyond your starting allotment. If you want to win the game, make your army capable of taking objectives. Thus, a flyer, in this sense, is like any other unit- it either is helping you take objectives from the enemy or it is a waste. More so than anything else, though, flyers fall into this pattern; their ability to cross the board quickly and (in many cases) deploy significant firepower as well as deposit a unit with precision make them perfectly-suited to surprise-claiming an objective at the end of the game.
Thus we have the list of the “good” flyers so far: Vendetta (and to a much lesser degree the Valkyrie), Night Scythe, Stormraven, and Heldrake. You may notice a pattern there- most all of them are transports, most all of them are AV12, and most all of them are capable of bringing down other flyers. These are no coincidences; since there are so few Skyfire weapons in the game, flyers need to be able to deal with other flying targets effectively, as anything else is unlikely to do so. Since most actual Skyfire weapons are S7 or worse (which is to say the Quad Gun and Flakk Missiles as well as Vector Strikes and Tesla), AV12 provides a significant gain in protection compared to AV11 or AV10, essentially doubling or tripling your survivability.
Most important, however, is the ability to transport a unit. While they might be able to mitigate the firepower of a flyer even if it does stay alive and not every enemy will have flyers for you to shoot down, the ability to drop a troop squad on an objective will always be relevant. Even with their limited turning arc, flyers can easily be in a position to threaten 2-3 different objectives, and very few armies can realistically protect that many positions simultaneously, especially if your army has other firepower left over; they might be able to put a squad or two on each objective, but with the firepower of the flyer plus the unit inside plus anything else you have on the table, clearing at least one of those objectives should be entirely achievable.
(This, incidentally, is why the Heldrake makes the list as the only non-transport; it can so consistently and irrevocably clean out units of virtually all kinds that it gets the nod despite being unable to actually claim the objective itself. Whereas other flyers can be avoided or ignored, the Heldrake cannot- rare is the army can simply suffer through 6-10 automatic hits that ignore cover and armor every turn for the whole game, much less when multiplied by two or three Heldrakes.)
The other passable flyers (Doom Scythe, Stormtalon, Razorwing, Dakkajet) are all more mitigatable because they cannot drop troops on an objective late game (which is, you’ll remember, often going to be a 6VP swing as you take it from your opponent and claim it for yourself) and do not have the ground-attack firepower- or, in most cases, the anti-aircraft firepower- to make themselves impossible to ignore. While they may certainly be dangerous in their own right, their maneuverability and fire arc issues mean that good positioning can often deny them shots against particular targets for significant portions of the game. The Stormtalon can bypass this to a limited degree with its turret mount, but since most targets are still getting armor/cover saves against it (and you still have to roll to hit, etc) it doesn’t have the same infallibility of the Heldrake’s weapon. Again: these are not necessarily bad units, but they are more vulnerable to being destroyed and less able to affect the victory conditions of the game as compared to the top-tier flyers. In armies that have access to them you still probably want to take them because flyers are powerful tools, but they are not defining the course of a game the way the other units do.
Understand, even the good flyers cannot win games by themselves- running six Scythes or Vendettas is, on its own, not a plan that will be able to beat a strong opposing list. But when backed up by solid ground forces that can claim your own objectives and apply efficient firepower to the enemy, they can be very scary to the other general. Good flyers exist to support and deliver ground forces, not to singlehandedly win the game on their own. Having a huge number of flyers on the table is usually just asking for maneuvering issues, but having 2-4 of them cruising around unhindered means you can bring a lot of guns to bear on targets and deny those targets easy cover saves. Flyers are great ways to deliver firepower to hard-to-reach enemies, whether they are hiding behind terrain or because they are simply too physically-distant for your other guns to hurt them. They can do a lot of damage to these backfield units, but ones sitting in midfield or advancing into your side are going to be a lot tougher for them to bring down, as they will get fewer turns to strafe them when coming onto the board and their turning radius will be more limiting, as flyers’ attack patterns tend to mean that they want to hug board edges.
To reiterate: the most dangerous flyers in 6th Edition are the ones that can swing the objective game for you, either by clearing enemy units from one of their objectives, by depositing your own units on said objective, or both at once. When you are looking to write a list and wondering if it can handle flyers, look to these units as your baseline- do you have good ways to stop them from dropping their cargo off with impunity? Can your anti-flyer weapons regularly get penetrating hits against AV12? Are those weapons resilient enough to suffer the flyers’ alpha strike if they don’t have Interceptor? Especially with the introduction of the Heldrake, you need to be able to answer these questions for your list or you are going to struggle greatly against any army that brings an airborne contingent- and you can expect the number that to continue to rise so long as GW keeps on its current course.
