The Husbands of River Song was basically Steven Moffat doing Hitchhikers Guide to the Galaxy. Which isn't necessarily a bad thing. It certainly makes a nice change of pace from his usual approach to the Christmas specials, where he gives a standard episode plot or idea some generic Christmas lip service. It also makes a nice change of pace from his general approach of either writing about time travel or aping Robert Holmes (or both, of course). It was Moffat's best Christmas special1. He kept things simple, aimed for fun (he even managed to hit it a few times), and didn't shy away from mentioning Christmas.
Although that's not to say it some sort of flawless masterpiece. It wasn't. This paled in comparison to even the worst RTD era Crimbo spectacular. For my money that's The Next Doctor, though whichever one you pick as least good it's going to be better than this. Even if you're really into Douglas Adams this episode probably wasn't better than Insert-Name-of-Your-Least-Favourite-Davies-Era-Special-Here because it wasn't enough like Hitchhikers Guide to be satisfying.
The other thing that could save this episode for some people is the presence of River Song. The thing is, most people aren't going to be into River Song to that extent. She's a recurring character whose last appearance came two and a half years ago, in The Name of the Doctor. The show should hopefully have attracted a few new viewers by then, but no concessions were made to them. There was no explanation for who River was or why we should care about her. Even people who follow the show enough to remember her could probably have done with a refresher on her history2.
The episode's central problem is that it assumes everyone's familiar with River's convoluted character arc and is happy to see her back. Which isn't true. I've nothing against the character or Alex Kingston but I'd be perfectly happy if neither appeared in the show again. Self aware jokes about flow charts are fine and fun. The final scenes all revolving around things not mentioned since Silence in the Library (seven years ago), with no explanations on offer for anyone who didn't follow the references, was too much.
In fact I don't think it would be a stretch to say that anyone who's come to the show since Capaldi joined, and there have to be some people who that's true for, would have been nothing but confused for large chunks of this story. Even when her relationship to the Doctor was revealed there was still a that stuff with the sonic screwdriver, and the Doctor's haircut and suit setting up Silence in the Library to contend with. This sort of approach is the reason Moffat needs to leave the show. He's a fine writer when he's producing one story a year but his approach to running the show is close to being actively harmful at this point. Give it to Mark Gatiss or Jamie Mathieson or Chris 'Chibbers' Chibnall or Sarah Dollard. The next series really needs to see Moffat bowing out gracefully.
Dragging things back to The Husbands of River Song... the show's other main guest stars were Greg Davies, best known as angry teacher Mr Gilbert on The Inbetweeners, and Matt Lucas, best known as one of the lads off Little Britain. Greg Davies played a megalomaniac king detached from his body for the first half of the episode and was very good. In fact Hydroflax could have been a really bad character had they not had someone as good at producing the yucks as Davies. Matt Lucas wasn't afforded such a memorable role. He wasn't especially good, but nor was he especially bad. He was simply there. His part could have gone to a compete unknown and no one would have batted an eyelid.
As I said in pretty much every review of series nine, Capaldi and the set design team were very good. Capaldi did a great job, elevating generic lines about River Song continuity into something watchable. Not for the first time I'm pleased we had him in the show over Matt Smith (although this is one of the few examples of an episode of Doctor Who that could not be rework to feature any other Doctor). The design team gave us a nice model shot towards the start (Hydroflax's spaceship), subtly reused Trap Street from Face the Raven as an alien world, and did a pair of nice spaceship interiors (although technically one may well have been a location shoot). They made it easier to gloss over the fact that this was an excuse for Moffat to revisit River Song as a concept again, something he swore he assured us he was going to stop doing after her previous appearance in 2013 (see here for one of many possible examples of that). Didn't stick to that, did he?
1 I'd like to point out it had staggeringly low expectations to meet though, so this shouldn't be taken as any sort of worthwhile achievement.
2 Here I'm taking about people who watch every week, or most weeks, but don't obsess every details of the show. Despite Moff's assertions to the contrary they still make up the bulk of the programme's viewership.