It does not have complete feature parity. One of the things missing, that is important for me and the team I am on, is the ability to share screens and allow for remote control in that session. That is a Mac App only right now, at least the last time I checked.
Now that screenhero is no more, screen sharing is literally the only reason one of my teams use slack. We use hipchat (not by choice) for the rest of our communication.
For me, cmd + tabbing to a chat application is a much better user experience than having to dig through open tabs and/or multiple instances of the browser.