Hmm, can you provide more details about the xfinity functionality? As of a week or two ago I wasn't able to find any support for it beyond just generically casting the tab from a PC to chromecast.
I love using HBO Now on my Chromecast but man is it a bear to fast forward and reverse using the iPhone app.
It's not any better on Android, and Hulu isn't any better either (but they do have an ad-free subscription now, which makes them actually nice to use finally).
I don't know why they don't just add time input dialog box on all video apps. Scrubbing just isn't cutting it