Phantoms were the most popular mob design from the mob ballot contest, and while their in game design had a rocky start, what we have now, atleast visually, is something arguably better than what we voted on. The Phantoms are the Overworld's first flying mob, and fairly dangerous to boot. However, Phantoms, as a mob, seem to be problematic.
So let's talk about what they do. They spawn quite frequently after the third night of no sleep, and they on average do 4-5 hearts of damage, enough to kill an unarmed player in two shots in some cases. This is on par with the Overworld's strongest mob, the Enderman. However, unlike the Enderman, they're neither naturally passive until provoked, nor have a natural weakness, meaning more often than not, the player HAS to fight them.
And I mean HAS to fight them. Phantoms can fly faster than a player can run, and are the only mob to outspeed a running player. Even diving in water, which hinders other mobs and is usually used as a getaway option for the player, doesn't work on Phantoms as they have no slowdown in water. Not only that, but Phantoms have rather small hitboxes that makes hitting them in Melee fights kind of difficult to do without getting hit yourself. For perspective, the game assumes that the player:
Has a good set of armour, iron or more
Access to a bed that isn't out in the open
A ranged projectile weapon
There's a few problems with this, however. Phantoms start spawning an hour after the player first spawns into a world, and not all of these things can be obtained within this time. The player, if lucky, will most likely gain access to their first piece of iron within 20 minutes, and while possible to find 24 pieces of iron within an hour, a player is most likely gonna use these for weapons and tools before armour.