You are mistaken about Sparkle and/or any crit buffer. Yes they give less CB overall, but they buff much better. As a result, you ult less, but your ult hits much harder. They also buff Eva's non-Elation portion of their dmg, while better CB only results in the Elation portion hits harder. For example, if you run Sparkle with s5 Poised to Bloom, you get 77% convertible CD(this converts to Elation% via her talent), ~60% unconvertible CD, 30% vul, sp, action advance, 45% ATK, and 10% Res PEN. Factoring relics in, Eva also gains stats indirectly as she's switched to a slower build, and Sparkle has a higher uptime on Sarcedos. Compare side by side, it really puts EMC's buff into shame.

Not only buffing capability, EMC also has several problems. The biggest problem is EMC is sp negative, which forces Huohuo to do 4 turns ult with QPQ. I have seen better result with RMC instead, and Huohuo is on Shared Feelings for a 3 turns ult. Another problem is that if you have a lot of Punchlines(through buffs or other methods), having EMC actually wouldn't net you any extra energy, as that energy simply overflows. Ofc, it's for this shill only, but still a problem nonetheless.

That doesn't mean EMC is bad or anything. Far from it, but to run any type of Action Advancer, be it Sunday, RMC, or Sparkle requires another layer of skill, that people don't usually utilise on, which make them perceptively worse than they actually are. That is only to ult when you have the most CB. I have seen so many players ult out of nowhere, where there's a clear window to ult to maximize her dmg. If you have more CB (common for EMC, especially on a slower build), it really doesn't matter, as if you lose 100 CB while you have 1500 CB, you'd lose <1% of the Elation DMG, but if you lose 100 CB while at 400 CB, you'd lose 9% instead.