I basically like it. I wouldn't use it unmodified (chunkosauruswrex's concerns about chickens are valid--but easily-fixed) but it leads in an interesting direction. If you want to go in a really interesting direction you could track each kind of XP separately, and require at least 25 XP of each kind in order to level up. (So, 50 combat XP and 50 social XP = no level up until you gain 25 exploration XP.)
As with last month's UA, the real value in this UA isn't so much the rules provided as the encouragement it gives to DMs to think outside the box. I hate milestone levelling and will not use it (it completely misses the point of level-based systems, which is all about operant conditioning and Pavlovian rewards), and I've already messed around with my own XP tables, but this UA has me thinking once again about new and different XP tables and ways to award XP.
For example,
"Earn 1 point of XP per game session. Every 10 XP earned gives you a level. A player or the DM may also reward XP to another PC who does something particularly awesome, and/or in keeping with their bonds/flaws/ideals."
I wouldn't have thought of that method without this UA, because this UA very pointedly makes the point that the semi-exponential shape of the XP tables in the PHB are not important to the design of 5E as a whole.