What's funnier is that Dendro being so strong is most likely unintended, like, people say some characters were already made for it, but I doubt hoyo actually has that great foresight as some claim. (Long text incoming)
One could argue Kuki, due to timing and (low) EM scaling, is working as intended, but considering how much they tend overvalue utility (the reason Dehya is in the state she is, Zhongli's release state and Kokomi's -100% CR, to name a few), I highly doubt it, being healer while at the same time doing most of the damage of the team doesn't seem like something Hoyo would do, specially not for a 4* and oh boy do they love releasing bad ones, they probably intended her EM to be used for proccing crystalize shields for Itto or something.
Yae, due to having some EM scaling as well, could also be argued that works as intended, but what lets her function is something that they deemed a bug and had to revert changes after fixing due to 'feedback'. Even then, Fischl works better. I dare even say she might have been intended to be standard at some point (released at 2.5 and kit didn't seem intended to be as good as it is rn) but chose not to.
Lisa, who ascends with EM, could also look like she was intended to work with Dendro, however, she's a starter character, and just like standard banner characters, those aren't usually intended to be good in gacha games, so that was probably given to her as a useless ascension stat that seemed useful to newer players, but not so much once you played enough, giving you yet another incentive to pull.
Cyno, who was released after Dendro, and scales with EM should have been really strong with Dendro reactions right? well, he works, but is still outclassed by Fischl and Keqing. One would believe they would have learned from those two and made Cyno stronger but they didn't, while Tighnari and Al'Haitham don't have the same icd issues as him... Another victim of colorism?.
Raiden EM is a thing, and it is played ignoring her burst, which her entire kit is designed and centered around. Definitely unintended.
Thoma and Xinyan working for burgeon is a miracle for them, no EM scaling on either.
Fischl and Kequeen, whose amounts of electro application and icd's were probably not taken into account when designing how aggravate and quicken work, are probably the most benefitted ones. two are 4* and the other is a standard 5*, therefore most likely unintended.
Razor working better with EM than Phys is just the icing on the cake. Probably the reason why Mika exists and isn't in a Eula banner (apart from being awful to actually pair him with her on a team) is because they want to offset this somehow, if Razor and maybe also Rosaria come on a banner close to Mika's, then I'll take it as pretty much a confirmation of this theory.
Hydro characters barely benefit from it, but from those who do Barbara was nerfed and Mona's C1 wasn't changed to include bloom, Candace being post-dendro is meh (Yet another victim?) the others might be working as intended as there's not much to exploit from it, other than Xinqiu's and Tartaglia's already known insane elemental application, which we all already know were also probably unintended.
Fridge is kind of a meme in which only Diona benefits from it. At C6. Yeah.
The only certain intended behaviour is probably burning, which they nerfed before giving it to us, probably because people were already theorizing with it's use for melt teams, and as of right now the only one who can use it easily is Nahida, with C1 and/or another pyro applicator, because of course...
TL;DR: They shot themselves in the foot when designing Dendro, and most strong interactions with it are probably unintended.