Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Romsca]

And also, barrage uses your auto attack on the bow, which is 52. Pretty sure pets scale off summoner weapon damage which for AIW grimoire is 81. Honestly it is late for me. I will take a look at it tomorrow when servers are back up.

[Viridiana]

And if it was only a single target attack that would be fine. I don't see black mages popping flare on a single target and that's 260 potency times 5 targets. Does astral fire boost that? I don't know. I am just hesitant for it to be on a 2 minute cool down. Spurs description says "increases pets magic attack power", rouse says damage.

Yes, but if we're talking about helping in FCOB, then it will more often than not be single target just because there won't be much opportunity for AOE. Who cares if it means SMN can speedrun dungeons better?

And also, barrage uses your auto attack on the bow, which is 52. Pretty sure pets scale off summoner weapon damage which for AIW grimoire is 81. Honestly it is late for me. I will take a look at it tomorrow when servers are back up.

Barrage is 6 or 8 auto attacks over 10 seconds. Each IAW AA (or Yoichi, which will be BIS soon) is 101 potency. And yeah, they both scale off of their respective weapon damage stats. The real question is the equivalency between Ifrit potency and BRD potency, which would have to be tested at the same Int/Dex, Det, and weapon ilvl.