Sauron Cultists on Rings of Power Are Nothing More Than Bad Storytelling
One look at the Internet would tell you, that there are plenty of reasons to criticize The Rings of Power for various storytelling problems.
Yes, even after you discount all the critics who specialize in rage-views, and even if you treat the show as if it was its own thing, and not based on Tolkien's legendarium. And one of these reasons is the three Sauron cultists, plus their role in the narrative.
First, "the Eminem Lady" and her gang were just not especially impressive, however you look at them. Given their screentime, and the prodigious magic power they had at their command – in the world where any sort of actual magic is supposed to be rare, special and impressive – they have failed to establish themselves as villains in their own right, before getting hard-pressed by a small bunch of rock-throwing folk, and then easily one-shotted by the Stanger, once the latter got his wits together.
But more importantly their primary role in the plot was just staging a fake Sauron reveal, 15 minutes before the actual Sauron reveal. Their existence could have been more meaningful, if only the fake reveal had been even slightly convincing. But it wasn't. Not to 95% of the Internet which had their fingers firmly pointed at Halbrand as Sauron's real form all the time – it is not like foreshadowing that Halbrand is Sauron was subtle – and predicted that the Stranger is almost certainly going to be Gandalf, or, failing that, one of Gandalf's fellow wizards.
So, the three cultists were pretty much non-characters by themselves, just a bunch of CRPG mid-bosses without distinctive personalities –they used a suspiciously CRPG-like variety of magic powers too – and their primary function in the plot was meaningless.
Their secondary function of forcing the Harfoots into the situation where the latter ultimately chose kindness over ruthless pragmatism, thus reminding the Stranger who he truly is, could have been accomplished by a blind force of nature just as well.
The only saving grace of their appearance was their final showdown with the Stranger, which was actually a better and more appropriate example of a magical battle that all the telekinetic punching from Peter Jackson's movie trilogies. Oh, by dying they at least established that the Stranger is the real deal, as if the meteor scene hadn't tell us that already. But that was not enough to justify their existence.