Ok so lets test this out a bit.
So on a normal round, a TWF fighting ranger at 9th level with 20 dex is doing lets say 1d6 + 5, 1d6 + 5, +1d6 (for hunter's mark). Lets also throw in a 1d8 damage for the hunter's giant slayer as a "default" option for a ranger. if we assume the old 65% hit rate, than 25 damage * .65 = 16.25 DPR. I normally do 60% myself but since crits are harder to calculate here with all of the floating damage I'm bumping it up 5%.
Ok so conjure barrage does 5d8 damage (22.5) or 11.25 on a passed reflex save. Lets be mean and assume the monsters will pass their reflex saves 65% of the time (so I'm really giving advantage to my attacks here). So that 15.2 DPR per creature hit. (note I'm not actually doing that much less damage per creature than my normal routine).
So just hitting 2 creatures I'm already doing 87% more damage than my normal routine. Make it 3 (which in a 60 foot cone where I'm not worried about friendlies is not that hard), that's 181% more damage.
I mean if your arguing that the spell isn't fireball.... ok its not fireball. But it is a significant boost to the rangers damage in several common areas.