Forrester said:
The problem is that it's a touch attack, so it's going to hit 95% of the time when cast by the average Trickster. And if he's Hasted, he can cast three of them a round (might as well cast one Quickened one, it's only 4th level) -- ignoring the Twinned Spell nonsense for now -- and do an automatic massive horde of damage.
What's wrong with an "automatic massive horde of damage"?
Consider a rog4/wiz5/arcane trickster 2, flying, hasted and improved invis'd. They get BAB +6, +3d6 sneak attack, and are caster level 7th. Three rays of frost in one round equates to 3d4+9d6 damage, and they've likely blown all their 4th level spell slots for that (improved invis + quickened RoF).
Consider a rog11 with a bow, also flying, hasted and improved invis'd. They get BAB +8/+3, and +6d6 sneak attack. Assuming Str 14, Dex 20 and a +3 bow, they have +16/+16/+11, and do 1d8+5+6d6 damage per hit. With Rapid Shot, they have +14/+14/+14/+9, and have a good chance of hitting at least twice. That's 2d8+10+12d6 damage.
Consider a wiz11, also flying, hasted and improved invis'd. They let off two fireballs and a quickened magic missile. That's 20d6, save for half, plus 5d4+5. If the target makes all their saves, that's still 10d6+5d4+5.
All of these seem fairly comparable to me.
At the top end, consider a rog4/wiz5/arcane trickster 10, who is for some reason still using ray of frost. They get BAB +10/+5, +6d6 sneak attack, and are caster level 15th. Three RoF's in one round equates to 3d4+18d6.
Consider a rog19. They get BAB +14/+9/+4 and +10d6 sneak attack. Assuming Str 18, Dex 24 and a +5 bow, their attacks are at minimum +26/+21/+16, and most likely they'll be at least +30/+25/+20. Add in haste and Rapid Shot for +28/+28/+28/+23/+18. If four attacks hit out of six (highly likely, given how AC tends to increase more slowly compared to BAB), that's at least 4d8+32+40d6. It'll almost certainly be more, if you allow for energy enchantments on the bow, bracers of archery, etc.
Now consider a rgr3/ftr6/deepwood sniper 10. Minimum attack bonus for this character will be on the order of +35/+30/+25/+20. Haste and Rapid Shot make it +33/+33/+33/+28/+23/+18. If five attacks hit, and one of them is a crit (recall the DWS's crit range is 18-20), that's something like 9d8+135, again plus energy enchantments.
Again, all of these seem comparable to me, assuming the arcane trickster has other tricks up his sleeve than just the ray of frost thing.
I also haven't bothered to minmax these examples out the wazoo. I'm sure the ultimate munchkins on this here UNseboard could do better.
At least with an arrow you have to make it through armor.
AC is generally not a problem for archers, what with stacked bow and arrow bonuses, bracers of archery, and whatnot. The exception is big boss monsters like dragons, but these also usually have ways of dealing with invisible people.