So about Summoner...

[Sunako]

If you need a singletarget fight, then have a look at Kefka for compare as well. BLM > SAM there.

Not only adds affect the efficiency of a job, downtimes and movement mechanics affect it as well.

SAM needs uptime on the boss because he has nearly no delayable bursts and higanbana suffers a lot when the boss disappers early after application. Buffs run out too if the boss is untargetable for too long time.
MNK has a similar problem. That's why MNK and SAM do so well in O6. No positionals, 100% uptime.

You simply can not balance for all kinds of bosses.

And the crazy ADD dps for top SMN on O5 results from delaying bahamut trance and keeping 2 etherstacks and then to combine 2 painflare with deathflare (start trance and end it immediately) and then call bahamut and enkindle.
But if you don't "cheat" and use the skip strategy the SMN will get problems with his ghost in the passenger cabin. Because there is nothing left he can use. This is very specific to that single fight.

Ya in faust kefka samurai is bad because slashing debuff fall off skill speed buff fall off, damage buff fall off 3 times while blm have infinite enochian and BLM can collect free fouls. If you compare god kefka. BLM is behind samurai again because fight have no downtime. And yes that is true, it is impossible balance all fights for all jobs. You would need balance every job after every raid tier based how much downtime, aoe or singletarget fight have. Sound pretty much impossible.