Update:
We went with Level Up: Advanced 5E, with the option for players to use 2014 if they were more comfortable with that - or if the options weren't available in LU (such as many of the Eberron-specific options).
The campaign lasted about 3 months before it ended with an unceremonious TPK. I had high hopes, but alas, it was not meant to be.
Here are some issues we had, specific to Level Up.
- I was the only person who owned the books, so no one really got familiar with how their abilities worked (despite my having shared the online tools with them.)
- The massive book was difficult to navigate and just ... handle. One of the players repeatedly dropped my copy. (Multiple times a session.) [On the plus side, it's only a little scuffed - whereas a WotC book would be disintegrated.]
- It's not really "compatible" as I understand compatibility to be. Trying to go from different versions of D&D (2014, Level Up, Flee Mortals, and OAR's converted B/X adventures) was just a mess.
- If you're going to run B/X converted adventures, you better use characters designed to be thrown away. There's way too much time sink required to make characters in Level Up.