Arrows are consumables. And if you follow the standard rules for consumables, very expensive to consume; they cost half as much as a non-consumable item of the same rarity!+1/+2/+3 DC could be very strong, it depends on the spells and could require attunement, just like the spell recovery.
The stacking also happens with ranged attacks with a bow +2 and arrows +2 You could even add a Giant/Dragon slaying into that mix.
As for melee characters vs spellcasters. A Ranger vs a Warlock: Not a great fighter, but can cast some spells vs. not a great spellcaster that can do decent amount of damage every round with cantrips.
Every 2 attacks with a +1 arrow you are throwing away a +1 magic sword. Finding replacement +X arrows is difficult if you use random tables, and expensive if you buy them at 250 gp per +1 arrow (!).
Even if you make a random +X ammunition (a) produce ammo the PC wants, and (b) give out 3d6 shots, you are lucky to find one such cache of ammunition your entire adventuring career, and each shot is used up and gone for good.