Setup: MacOSX 10.10.1, Endnote X7.2.1 Academic version, Macbook pro Retina
Most of the problem is about the Sync which I started to like when it functions right. Unfortunately, it is the one causing multiple problems.
Proxy: It does not work well through proxy (username+password and many of the ports blocked) used in our institution. It will not work at all when you set it up, but if you do the setup using a direct link first (mobile phone in Internet share mode) then next time onward it will sync (most of the time).
High cpu use during sync: While trying to sync cpu use increase exponentially from negligile to 120- 140 % as long as it is trying to sync. It is more likely to get unresponsive during this period.
Temporary solution. 1. Use a internet connection without proxy during setup. 2. Switch off the automatic sync. and sync during startup options in the preferences panel. 3. Sync. manually when you are not doing anything else on that computer.
With these options, it behaves nicely with very little cpu use.
Missing references. If for any reason the internet connection get disrupted during sync. and as a result few of the citations were not synced then next time onward they will never show up in your library. In this case one have to delete the library and do it from the beginning with a fress library.
Hopefully this problems are looked after and future versions work more predictably.