Kieron Gillen is in a bad position as writer of "Uncanny X-Men" #13. "Avengers vs. X-Men" is still rampaging its way across the Marvel Universe and as a result, few people are going to pay much attention to anything in "Uncanny X-Men" that isn't connected to the event. The solution, of course, is for Gillen to write issues of "Uncanny X-Men" that tie directly into "Avengers vs. X-Men" but don't have a huge effect on that mini-series.
In other words, it's not an ideal situation, and one that isn't terribly likely to turn out a great issue of "Uncanny X-Men" for the ages. In the case of "Uncanny X-Men" #13, Gillen and Billy Tan do their best, filling in some small details about the back-story of the Phoenix while also continuing Gillen's plot about the robotic creature called Unit. Gillen uses the comic to try and explain the purpose of the Five Lights (although the other two Lights that since appeared in "Generation Hope" are quietly ignored), which X-Men readers will no doubt appreciate, and it certainly casts a bit more light on the events of "Avengers vs. X-Men" #5.
It's a little odd in places, though. There's a big "we can't have this actively affect our big mini-series" hand wave near the end of the issue, and the villain Unit is still about as interesting as its name (which is to say, not at all). While I like the idea in theory of the not-on-the-moon X-Men characters hanging out and commiserating over being left behind, it comes across once again as something that's unessential and filling space in the name of being an "Avengers vs. X-Men" tie-in.
Tan's pencils are fine if nothing out of the ordinary; he's good at facial expressions of sadness and disappointment, both of which are pulled out on a regular basis, but backgrounds seem overly simplistic more often than not. I do think he draws Unit in a creepy manner, which does more to sell the character to me than anything it's actually done up until this point. (Adam Kubert's cover has, needless to say, little to do with the contents of this issue.)
"Uncanny X-Men" #13 isn't bad, but it's not that fun either. I feel like writing these tie-ins is a bit of a thankless task right now; Gillen and Tan are making the best they can of a bad situation. Nonetheless, "Uncanny X-Men" has been a lot better than this over the past year.