I have to admit I have no idea what the title of the episode means. I know the place they're in is called Tsuranga, but what was the conundrum? But whatever. I enjoyed the episode a lot.
It started very strong, with the Doctor partially disabled and fighting her pain while she tried to figure out where she was and what the problem was. Astos was fantastic but sadly short-lived. The passengers on the ship were working against a tight deadline and faced with an unkillable foe. The dialogue was strong, and I absolutely enjoyed the Doctor being uncharacteristically hostile in the beginning and then, when it was pointed out, realizing what she was doing and apologizing.
I was also pleased with the focus on the guest characters' stories, something that DW used to do so well in the classic series and the RTD era, and we haven't seen for years. The episode juggled three different sets of stories - the general and her brother, the pregnant guy, and the medic - with varying degrees of success; I would have liked to have seen more of the medic and her struggles.
The only part of the story that dragged was Ryan's, and that was mostly because of the way it was crafted. His issues with dealing with his father were brought up during a rush to quickly gather all of the passengers on the very large ship in one place, and he and Yaz stop to have their heartfelt conversation. This scene immediately dispelled any sense of emergency, and it didn't help that Yaz's entire role here was to prompt Ryan with questions so that he could provide exposition to the audience. Why wasn't this exchange done while they were running? The urgency would have given Yaz an excuse for being blunt, and the physical exertion would have allowed Ryan to express (and vent) some of his anger and grief. And the scene wouldn't have killed the episode's momentum.
Oh, and the anti-matter speech. This is the second time (at least - maybe third) this season where I've felt that the inclusion of more real-world science and/or history caused the Doctor's speech to run too long and feel like a lecture.
I did really like the pting. It was a bit ridiculous, eating everything and never getting any larger, but it suitably mysterious (was it actually intelligent?). I also enjoy storylines in which we assume the motivations of the antagonist and then find out we were wrong. I suppose the fact that it's actually eating energy explains its lack of growth, if we assume that when it eats matter, it's converting it to energy.
I realized later that this episode was a retelling of "42": a spaceship that cannot escape the situation it is in, beset by a creature that cannot be defeated, and facing an onrushing deadline of destruction. In general, I don't think it compares well. "42" only had two character stories to focus on - McDonnell trying to hide her crime and Korwin, and Martha and her mother - rather than four, and the only time the narrative slowed due to one of them - Martha realizing she was going to die without her family ever knowing where she went - the characters actually had nothing else they could do.
(Caveat 1: That scene where the Doctor screams, "I'll save you!" silently into space over and over again did completely kill the momentum, especially since the Doctor was completely aware that he was wasting precious seconds.)
(Caveat 2: "42" is one of my favorite episodes, so I am biased. I know most people don't like the episode, so your mileage may vary.)
One last bit: This is becoming a bit repetitious, but I'd like to see more of the companions doing things on their own, figuring things out and making decisions. In this episode, the Doctor figured everything out and told everyone what to do. Yaz did a great job dealing with the pting at the reactor, but it was the Doctor who sent her to that position. Also, Graham and Ryan helping the pregnant guy reminded me of Tegan and Turlough in the ventilation ducts in "Terminus" - let's just send them there to get them out of the way of the real story. Yes, Ryan actually had a character development reason to be there, but it felt tacked on.