The pacing in the episode was massively off, and it didn't help that it felt like a 40 minute version of the Pond Life shorts that were aired before the season started. Just suddenly, here's a bad guy, here's an explanation, here's a resolution! This happens a lot with Who, where the episodes would benefit from an hour running time, but it was seriously noticeable this time.
I wouldn't be surprised if it's more the case that Pond Life was a little five minute version of the Power of Three, rather than the other way round. Written by Chris Chibnall, similar vibe - I reckon they looked at the finished episode and said to Chris, "Can we have some more of this, please?"
Makes it a little unfortunate that the episode proper came after, but it was a fun little piece. There were a
lot of bits that didn't hang together - the orderly twins were unexplained, the abductions in the hospital were unexplained, and the little girl who was sat there for a full year made precious little sense either - but I feel like the cubes were mainly an excuse to get the gang together on Earth for a little while. It's the kind of thing that's deflating on the first viewing, because it was a great little mystery, but when I was watching at peace with the knowledge that the resolution would be a little bit pants, I was fully on board for all the good stuff, of which there was a
lot. The ending is mercifully short, and contains enough good ideas swirling around in its goop that I smiled with it more than I frowned on the repeat viewing.
And honestly, I'd rather have "sonic screwdriver saves the day" - which we've not
really seen since the RTD days - than another round of "feeling an emotion saves the day." Fine in small doses, but series 6b has kind of over-gorged me on it.
On the subject of RTD, actually, there's a bit of symmetry here with Boom Town, from his first series. Which, again - first time I watched it, I thought "that was a bit naff," specifically because the ending was hasty and unforeshadowed. But like The Power of Three, Boom Town isn't really about Earth's invasion - it's fundamentally a character piece, a bit of down-time to let us enjoy the current TARDIS-team while we still have them, with the stuff about the rift thrown in more as an excuse to get them all together than anything else. And like the Power of Three, the episode is therefore a lot better on the second viewing when you know what you're getting than on the first viewing when you're naturally more drawn to the invasion plot and would be forgiven for being disappointed by the fact that it's a little bit thin on the ground.
Incidentally, while we're drawing parallels, Boom Town's bolt-from-the-blue ending served the bonus purpose of introducing an idea - looking into the heart of the TARDIS - which would later be significant to the finale. I don't think that's what's going on here, I think the Shakri are just a cool idea Chibnall had that he felt like throwing in, but I'm more than happy to add fuel to the speculatory fire.