I was also an early adopter of Google Voice, which is now heavily integrated with Hangouts, and the recent merger of the two services has made it even worse. When Hangouts is signed in on my desktop and I receive a phone call on my GV number it will often only ring on my desktop and not on my phone, and there is no way to turn this feature off.
Additionally, some of my friends have poor internet service and Hangouts handles that about as well as a toddler falling down an up escalator. Messages are frequently significantly delayed or dropped altogether, we often find ourselves asking each other what messages we have and have not received because our chat windows frequently do not match up.
There are few things more infuriating than having a messaging service tell me it has delivered a message when it actually silently failed to send it.