If you’re writing a D&D adventure, one of the things you’re probably not thinking about is “Are the characters using flying in this adventure”. Unless you’re doing an adventure for your home group, and you know they will be.
Most of the times when you buy a published adventure, it also doesn’t consider “do the characters fly?”
What effect does that have on the adventure? Well, yes, it makes a lot of floor-based traps irrelevant. But it’s likely that there aren’t that many of those. It can also give short-cuts to the end of the adventure. A bit rarer, and that’s not great.
But the #1 effect it’s likely to have is to make most combats irrelevant or boring. And that’s because D&D is a design focussed on melee combat.
I consider, in D&D, melee combat to be far more satisfying than ranged combat. The gameplay shines better, and it allows many opportunities where standing in exactly the right place on the battlefield allows you to control more area than immediately apparent. The fighters can protect the wizards and rogues, and you get incredible teamwork.
Ranged combat has a few major flaws. If you want to see a significant one, consider a wall that you can hide behind for full cover, with an archway with full visibility to the battlefield. Under D&D 5E rules, you can move 10 feet to the archway, attack several times against a foe, then finish your turn by moving back to full cover. If opponents try to use readied ranged attacks against you, they only get one attack per turn rather than their full attack sequences. It’s using peculiarities of the rules rather than anything else to give you an advantage.
D&D’s movement rules are predicated on melee combat. Other systems presume ranged combat first and are designed with it in mind.
And for most encounters, designers work them out with a melee in mind.
It gets worse, though, because many monsters don’t even have a ranged option. Or their option is limited in range. Sit 70 feet away from an opponent and pepper them with a shortbow, while they look at their javelin that always attacks with disadvantage.
And Strength-based monsters, even if they have a bow, have poor Dexterity scores so the damage is also poor. This applies to characters as well. While I suggest every character makes sure they have a ranged weapon, the drop-off in capabilities in 5E is significant. +9 to hit for 1d8+4 damage compared to +4 to hit for 1d8 damage? Hardly a comparison.
Now, why I’ve mostly concentrated on why flying characters cause issues, it’s also true of flying monsters. All the problems monsters have facing flying characters reappear when non-flying characters face flying monsters. Do you have ranged attacks? Are they effective at all?
I recently ran a fight involving gargoyles from the Monstrous Menagerie (A5E). These CR 2 monsters have a ranged attack (a thrown rock) and can fly and hover. And are resistant to non-magical weapons. Imagine your home group facing them in a large cavern where they never close to melee range. How many of your players can affect them?
Flying speed of 60 feet, so the proper tactics for the gargoyle are to close to 20 feet, throw a rock, then retreat to 50 feet away (upwards) every turn.
Used in one encounter, it’s a challenge that requires good thinking to overcome – I like that sort of stuff. If you do that in every encounter, then you very quickly invalidate a lot of character options.
The trouble with flying player characters is that it affects every encounter! (Monster encounters will tend to be more one-offs). It’s a design constraint that you have to consider again and again and again. And when the majority of groups are doing it on foot, having the group of all-fliers breaking the adventure and game consistently?
It’s an absolute pain.
Yes, you can work around it, but it becomes much more of an individual DM finding solutions, rather than there being a system solution. And, honestly, I’d rather spend time coming up with fun situations rather than dealing with this every time I plan encounters for a session.
You can design a system that presumes everyone flies. That works well. But having a group of fliers in a system where this is not generally the case? You’d have to massively redesign the game, and I don’t think that’s in everyone’s interest.
All of which goes to explain why I’m very wary of flying player characters!