My Nexus 4 phone is now often, not an actual phone, since I often lose phone audio until I reboot. This is a known bug introduced and not yet fixed with Lollipop, so it's a good reason to not upgrade.
Which is pretty poor, it's like the iPhone 5 (released slightly before the N4) not being able to run iOS 8. I mean it's a different price range, sure, but you'd expect to install software updates 2 years later without it breaking a core feature like making a phone call.