Flying means very little to most flyers, because readied actions interrupt them as they fly by and they get hit anyway. Or they run away. Resistance doesn't make an encounter hard, let alone extremely hard. And even melee fighters can hit well with ranged weapons for those few that are left.
How many teleporters can teleport at will farther than 30 feet? If the enemy is constantly running away by teleporting, how hard is the encounter? Not very.
That has not been my experience.
This is assuming a strength based fighter (as I've said, Dex based martials don't run up against these issues nearly as often). Typically, you won't invest heavily into Dexterity, due to heavy armor proficiency, so you're probably around a 12 in Dexterity. Thrown weapons have very short ranges, so if you're using them you're probably attacking with disadvantage. If you're attacking with a bow, you're effectively taking a -4 to hit and damage, but generally without disadvantage. Additionally, at T3, a fighter has 3 attacks. But a readied action only allows 1 attack. So if the fighter is forced to ready an action (far less likely with a bow), it effectively reduces their damage output by 66%, before any of the other "penalties" are factored in. That's a huge loss of efficacy.
With a bow, they probably don't need to ready unless they're trying to avoid disadvantage, but due to their lower Dex they still suffer somewhere in the ballpark of a 50% reduction in damage output due to their lower Dexterity, and that's assuming their bow has the same pluses as their primary weapon (admittedly, this assumes that the -4 penalty is meaningful and they don't have a 95% chance to hit after factoring in the penalty).
In the martial campaign I mentioned, one of the artifacts I gave the party was a magical harpoon named Ahab. Among it's many abilities, the primary one was being able to use the harpoon to make an attack that would either pull the creature to the wielder, or pull the wielder to the creature. The harpoon had an huge thrown range, and I added it specifically because that melee oriented party had a lot of trouble with flying (or otherwise highly mobile) enemies, when I played them intelligently (and as I've said, i don't like pulling my punches when DMing, because i feel it cheapens the experience). It was a very effective weapon and saw a lot of use in that campaign, despite none of the characters being built around using a harpoon.
As for resistance, that's half damage. While my PCs rarely run up against this past T1, because I make sure the martial PCs have magic weapons, I often allow them to recruit henchmen, who aren't guaranteed magic weapons (a significant portion of loot IMCs is randomly generated). The current party of 6 PCs has 3 henchmen (2 fighters and a barbarian IIRC). Let me tell you, when those henchmen don't have magic weapons and they're fighting a monster with resistance, the difference is noticable from behind the screen (I'm speaking metaphorically; I haven't used an actual DM screen in a long time). That's (over, due to rounding down) a 50% reduction in damage output. It's very noticable. When that's the case, those henchmen generally do very little to alter the trajectory of the encounter. It's certainly better than nothing, but in a lot of cases the tiny amount of damage they contribute means the monster would have gone down at the exact same time anyway (because the rogue or the warlock completely overkills the creature, rendering the henchman's damage contribution moot).
And if you have a creature that has both flight AND resistance (admittedly, fairly rare) in T3, such that you have to use a readied action to attack, you're potentially looking at dealing around 16% of your normal damage output, which I think we can all agree is crap. Fighters do deal good damage under optimal conditions, but not so much that 16% of their damage can be considered good, by any metric.
I'll freely admit though that that latter one can be considered an edge case. Even so, a 50% to 66% damage loss is a very significant reduction in damage output. Yeah, you can still technically contribute to the encounter, but you're probably not pulling your weight under those circumstances.