Here’s a good read on Reddit:
https://www.reddit.com/r/gamedev/comments/5l26if/6_months_later_this_is_why_were_migrating_from/
TL;DR: UNET/HLAPI/Lobby/MatchMaker/HostMigration had high potential, but due to lack of support of any kind and no plans for anything to do with UNET/HLAPI for 2017, these features are to be considered abandoned by Unity, especially when compared to how amazing their other docs/support are, by comparison.
Then to top it off, without host migration (a feature still advertised), if your host leaves, every one of your clients crashes along with them.
Unity is still an excellent product with excellent docs, support and tutorials, however, beware of UNET – it’s as if they hired a 3rd party to make this, then that person quit.
Everyone is in the dark and if the docs aren’t even updated 2 major versions later and not answering any Q+A, abandoned is a really appropriate term.