Case point, multiple Apple TVs on the same network, quite often broken. iTunes WiFi sync often breaks without a plug in and fiddle session, I guess they really want you to use iCloud.
App crashes on iOS are quite significant now where they used to be very rare imo.
Wifi is back to its old happy self with the last few patches and seems more stable, Yosemite early days it was a bear I will give you that.
What I have found is that some things just don't play nice with the OS. Openvpn is kind of a bear and I'm not sure what the underlying cause is, but its easy to kill and restart so not a major issue there.
I do regular heath checks and cleanup though. EtreCheck is a great way to get a look at all the cruft in your system and make some calls on what to clean out/up!