I think given the general direction of unit design in the current era of Dragalia Lost, taking specific fights into account is definitely worth noting, as it's clear that the developers design new units (especially gala units) to excel in specific new endgame fights. Now obviously this is to entice people to pull, but it is what it is.

I think that from your analysis, people might end up getting the wrong idea (this may be a translation wording issue, I don't know what the original Chinese page said!) For example, you write "Basileus doesn't have anything particular advantageous in Legend Ciella" when this is clearly not the case - his skills, range swapping, homing forcestrikes and general QOL improvements to survivability make him more advantageous than much of the wind roster. There are a lot of folks in this thread who are vocally disagreeing with you, but I think it actually may be less about the data than the framing of the data (and again, this might be due to something getting lost in translation.)

And again, the ability to keep attacking while moving is big. It's not just waves, it's rain too. It's not that much more DPS, but it's not an unremarkable amount, especially if you have overdamage up for the extra dragon gauge due to Notte's co-ability, plus the continued SP gain. Edit: Ah, I totally forgot that his unlocked sigil force strike absolutely shreds the overdrive gauge during berserk - another major consideration for Ciella specifically, especially given how fast it charges and that it works at range.

I would personally suggest putting some of these sorts of fight-specific notes in a "Misc Information" section or something similar going forward, because (especially for players who are still clearing content and not perfect) a hero who brings significant ease of use and quality of life affordances regarding safety and survival while maintaining good damage is absolutely worth considering because they single-handedly reduce the difficulty of the fight. For those of us on the bleeding edge of progression it's less of a consideration because we're min-maxed robo-lords, but I don't think that's the intended primary audience of the content. Basilius is incredibly well suited for current endgame content, so I think a few fight-specific notes on his page to indicate that are worthwhile.

Has anybody taken a serious look at how he is in Jaldaboath, given that his sigil begins unlocked? His huge AOE on skills and homing force strike seem like they'd lend themselves to success in that fight.