I don't know how to answer this.
I essentially treat 5e as a lighter, leaner version of 3e. Most of the adventures I've run in 5e so far have been converted from PF, with monsters redesigned according to an approach that tries to port 4e monster design concepts into 5e.
I don't use any core classes, all of the classes I use are homebrew written by me; I only use two core races, and homebrewed the rest (and even then I've tinkered with both said core races). A good percentage of my spell and various other power lists come from 5e, but a notable number are homebrew or converted from other editions (forex, one of my power source's power list is basically just a conversion of 3e ToM's shadow magic with a few extras sprinkled in). While I occasionally run into balance issues, for the most part my players haven't had any objections or concerns.
If there are mechanical "holes" in 5e, I'm probably blind to them because in my mind I'm just using 3e, adjusting numbers as necessary to account for bounded accuracy.
I couldn't tell you what's in the various 5e splats. I have no idea, and honestly I probably don't care: not because they're bad, but simply because the majority of the content isn't going to be relevant for me or my table. Similarly I have no idea what monsters come from what books, because I find 5e monster design uninspired ("claw claw bite" is boring, sorry not sorry).
Put me down as 6i, I guess.