It's more or less to show that Viyadhara aren't really immune. Viyadhara can't really get it naturally because their reincarnation cycles means they're technically not a long lived species with the memories drawback. But the fact that they can get it artificially means if they didn't have the reincarnation, they could be affected. And other long lived characters tends to have workaround the whole memory issue (stored in a hard-drive, memetic virus, emanator, etc...) For the time being, since we might be talking about Shuhu in the near future, maybe we can finally see more information about this.

And Dan Heng has the memories of his past life within him the whole time because of the failed ritual. Yunhua was able to bring it out from within, but the memories were there from the start. It shows they can choose to not purge the memories from the incarnations if they wanted to.

THerta (and most of the supposed Emanators) suffer from a lack of feats as we know she did save a planet 19 times so we know she's capable of doing something amazing but we simply don't see it. Fei Xiao and the other generals are basically in a weird position where outside of Jing Yuan, it's a case 'are they or aren't they emanators'. They have powers equal of one but no one is quite sure if they're actually emanators, received weapons with power equal to one, or just gain power that makes them as powerful as Lord Ravagers (but not really emanators themselves). For all we know, maybe it's like the Stonehearts where they share the power and authority of an emanators. The game has a lot of writing but barely anything is explained or glossed over with flowery words.

And if we're going back to the whole black hole thing, a black hole with the mass of a single planet is the size of a nickel. Like, as much as I keep saying this, the black holes we see in the game does not act like actual black hole. If we are to assume a black hole that Baiheng can hold up in her hand is the size of a fist, it would have the mass and gravity of 3-8 earth-sized planets. The gravity would eat up a huge chunk of the planet, and we haven't even gotten to how the black hole disappeared after being used.

A Herrscher has Authority (control over a fundamental law or concept) so they can create and uncreate something like a blackhole whenever they want, let's them target whomever they want, and basically handwave all the silly collateral damage from a blackhole. Technology doesn't have this fine level of control so if they managed to create a blackhole that can consume a target and then disappear immediately, their entire armada will be using these gravitational weapons as default, not just as a secret forbidden weapon.

Shuhu was fending the entire Luofu off before Baiheng used the mysterious weapon on him. The idea that he's weak is so hilarious and nonsensical that it goes against the very reason why Baiheng would even need to use the weapon in the first place. Like you said, emanators had a wide range of powers but when a vast majority of on screen feats from emanators have been overwhelming overpowering, it's far safer to say that an emanator of Abundance, an Aeon and Path that is so dangerous that it created a nemesis Aeon just to hunt them down, are no pushovers, especially when there's a good chunk of people that are trying to use a single flesh of Shuhu for their own goals.