It was the worst in the series unfortunately.
Yeah, it was complete sci-fi gibberish hogwash. Like, that shit is fine for Doctor Who and is half the reason why people are there, but leave it out of Sherlock. Eurus's "reprogramming" abilities are complete nonsense and completely unnecessary to boot. Like, it's so simple what they should have done instead of jumping the shark with that nonsense. Just give her the Holmes-family gift when it comes to deductions, but she uses her deductions to fuck with people and toy with them instead of helping them. Like, have her use her deductions to figure out something about the warden and use that to blackmail him instead of her mind-controlling the entire prison.
But no, they had to go with that. Sorry, being super smart doesn't mean you can literally bend people to your will and get them to do whatever you want. Like, I can accept mind games in particular situations to evoke particular feelings. Like using logic to make people super depressed is one thing that I'm willing to bend on, even though that doesn't really work that way either. But using logic to get people to literally do whatever the hell she wants, just because "she's smart" and that's it, including killing their families just because "she's an earworm and I can't get her out of my head" and the entire prison being at her beck and call somehow because of that same magical reprogramming ability that works for no other reason than "SHE ARE TEH SMARTEST AND YOU ALL DUMB! SHE IS ULTIMATE!!!"? Yeah, no. That's a bridge too far.
And then you have the whole directly lying to the audience with the plane segment stuff instead of finding a way to leave it ambiguous what's going on there. Fuck unreliable narrators.
And then you have stuff like the stupid bomb scene, which really makes no sense at all since Eurus had already given Sherlock and John plenty of reason to seek her out when she put him to sleep, so what exactly was the point of that scene (that obviously wasn't actually going to kill anyone that early in the episode) except to have a terrible CG moment when the two jumped out the window?
And then you have all the terrible Molly stuff, which they just drop completely abruptly and act like nothing happened there for the rest of the episode (and yeah, i know she briefly shows up at the end--that doesn't count and doesn't make up for just treating it like nothing and just leaving people hanging. If you're going to put stupid drama like that in the episode, at least do it right, do it with respect and don't just use it for a stupid 'OMG, THE DRAMA! THE DRAMA!" moment and then go "oh wait, never mind. We good now. Ugh.).
And the whole prison stuff just dragged on way too long in general. Yeah, we get it, Eurus has problems. You don't need to keep dragging this on to show us just how messed up she is. We got it after she killed that guy and his wife, and definitely got it by the time she dropped three guys into the ocean after that. Shit was just exhausting and had no purpose other than "yeah, you know that she's messed up, but you don't know just how messed up she is!!!!!"
Just trash pretty much the whole way through. Very disappointing.