Remember Pinkie Pride with the real-world footage thrown in? It immediately changes the character. By creating such a harsh, sudden connection to a different 'realm', the integrity of the intended realm is compromised. This is what's called "breaking immersion".
Said artistic tools CAN be for a specific reason sometimes, when the context fits, but to me it didn't fit in that episode.
It doesn't have to be that extreme though.
One example of it not being a problem is scenes where they use lighting effects or camera zoom and pan techniques - the latter even has been talked about in praise in Youtube videos. (I don't remember the episode with the scene I'm thinking of, but it was done in an elegant, subtle way, and served a specific technical purpose. It was not a style break, but a style enhancement.)
But lately there was another thing that immediately got me 'off-track in my mind', in season 5 episode 3 (Castle Sweet Castle) right after the intro, there was a zoom-in of what very much looked like painted, with tons of details, some kind of heavy blur filter, gradients and stuff. Well, I'll just post it here, then you can see for yourself - in case you haven't seen the episode. (Don't worry, no spoilers here.)
When I saw that, I lost attention for the plot of the episode, had to check again, was wondering why they would decide to suddenly put that attention-catching style break in there, was wondering whether I didn't pay attention and this was supposed to be a painting on a wall somewhere. (thus: immersion break)
Is there discussion about this specific scene somewhere? Or about the topic in general?