Then there are Konan's 6 Billion Tags, which will be fairly pointless against most opponents, seeing as the jutsu the Tags were used in, was fast and powerful enough to split apart a Lake. The tags were really put there to combat Madara's phasing abilities.
I'm pretty sure the addition of all those explosives would be a huge boost in firepower against most foes. I doubt the non-explosive form could drop a biju, for example, but I'd think with the tags it could manage just fine.
It's only more needed against Madara because the base jutsu is already overkill against most opponents 🙂
Really, we have a unique case here, and you labeled another similar but unique case with an asterisk without much hesitation.
ET and 6 Billion Tags offer a much larger advantage than Samehada's ability to increase its wielder's strength based on the strength of the opponent, but that's offset by ET & 6 Billion Tags needing ridiculous amounts of prep.
I don't think they're that similar.
Samehada's drain works against almost everyone. In some cases it provides more, in others less, but in general it does work on everyone, and on average it puts him in S, with a few outliers in both directions and about the same amount of outliers in both. In some cases he'll effectively be an S-, others an S+, and in the lion's share he'll be S.
Also, it's not prep based. Whether or not it can be done is an on-the-spot result of how a battle goes. All the * abilities are prep based.
Finally, it's not even the only one that greatly boosts the user's capabilities based on the opponents.
3rd Kazekage can steal weapons, Sasori can steal people with strings, Nagato's chakra rods can control people, and the mind-control powers are super reliant on the foe's strength.
I consider it "a conventional power with high but precedented range of variability".