Individual users throttling different limits for consensus-critical specifications can result in many, many unintended forks of the blockchain. Any time two nodes disagree on a consensus-critical issue (like block size), they will fork from one another if each continues to build on their respective chain. I wish BU luck, as I think most people realize by now that this is untenable for the idea of a single, global ledger -- hence its lack of devs and users.
BU currently has a variable called acceptable depth, which helps
nodes to stay on the longest chain. The philosophy is that none of the changes
in blocksize limit favored by majority come out of the blue but are
well anticipated by participants in the network.
The principle of converging to the longest valid chain is there,
but it has been extended from the purely algorithmic domain.
If BU has a grandfather, it is surely Theymos who was first to present
an idea of advertising block size preferences in transaction field.
As for number of devs, the quantity is not what matters here.
As for the number of users, I blame Classic for stealing the scene.
Bitcoin Unlimited is highly experimental and many of the basic operating principles are at best speculative. They should try it with an alt-coin first at least, or a sidechain.