OK, let's see. A secretly-entertaining-two-guests story. Not sure whether we had that before. Watching it didn't leave me with enough energy to investigate.
The problem became visible quite soon, too.
In such episodes that can be difficult to watch because so formulaic, especially for a season 7 ep, the little things provide the contrast. Spike exclaiming: "Fighting! Yes!" was very funny, and also Twilight's extreme portrayal towards Ember, or the reaction to the claim she is so much like Starlight.
I just personally don't enjoy this story archetype easily.
It did advance the character stories though, so that became the main driving purpose.

I found it personally disappointing that Ponyville folk is scared of pink fire breath.  I mean, seriously, how much nicer can you make a searing wave of air? Sneezing cotton candy? ... OH HEYYYY! IDEA!

What I found really good stuff was how Muffin Murderer  - I mean Ember - had difficulty telling Twilight and Starlight apart. Might seem absurd to us, but it happens in interspecies relations or even interracial. One is being used to facial features and general appearance of one's own kind and less used to distinguishing detail of others. That might have been hinted at here. Ember has a different perception, calibrated to dragons. Although execution in detail would then leave some questions, since Ember remarks how colorful everything is in Ponyville, indicating that she does have good color perception. Hm, maybe she just has trouble distinguishing different hues of purple. Hmmmm, but Twilight has wings and Starlight hasn't. OK, forget all that, I think she just has an attention deficit.

The coffee cup scene and possibly others before in the show indicate that the sun is traveling across the sky, really making me wonder why Celestia has to raise it in the morning if it's moving on its own anyway. I think this is just inconsistency and I find it a bit sad when Equestria is assimilated too much to our world's ways instead of celebrating the differences. (Twilight or Pinkie eating flowers seemed a rare case from long time ago underlining their... ponyness? Equinity? )

The charge scene between Ember and Thorax was so ridiculously time-warped that it has to be called a parody, but it didn't quite click with me. Probably reminded me too much of Naruto, LOL.

In essence, the topic of this episode was, again, severe neurosis. Don't we love our neurotic Equestrians? (Must be something in the water, considering Ember and Thorax acted refreshingly natural.) But it also showed how focus on positive outcomes can help with that.

The whole complex of lessons that emerged is beautiful though because in order to solve the friendship problem, Spike was prompted to change himself. He experienced the cause first-hand, him being the problem. So the message is that we create a better world by bettering ourselves and that being a teacher also means being a student.
And then Ember and Thorax help each other solve their problems, so this episode towards the end severely gained 'meaty bits'. Spike then learned, which should relate him to Discord, that sometimes you can be a more passive influence, less controlling, just a catalyst for bringing about the solution through crisis. Problems are teachers and people can assume that role. That's OK, too. ... As long as you don't make it your purpose in life, right?