I've been looking into that. My concern is the reliability of hangouts. How well does it work and what is the data & power consumption of it to be in constant wait?
The reliability of it working depends on the network (in this case, the Fi itself). Because of the nature of Fi, this depends on how well you get T-Mobile and/or Sprint signals. Having said that, since you were thinking to port the line in anyway, this should be irrelevant, as even if you could port the second line to Fi, then you wouldn't be able to make/receive calls if you don't get signals anyway. Also, I'm not sure how well Fi's wifi calling is, but in this case routing calls through Hangouts is basically setting that line to be on 'wifi call' whenever you're connected through wifi.
About the data and power consumption, it's actually not in 'constant wait'. Think about it just like any other messaging app (Whatsapp/Facebook/etc). It only rings when GV forwarded a phone call. The app is there ready to receive your 'Google Talk' and sms text anyway (if you set it to). There shouldn't be additional battery drain just for it to stand by waiting for phonecalls.
The only way I wouldn't suggest to do this is if you're expecting to do significant amount of important hours on the line. My concern is quality wise and battery wise. Personally so far I've experienced decent call quality, but my phonecalls rarely exceed 5 minutes each, and I'm mostly on wifi or very solid AT&T LTE connection (through Tracfone). I currently have three lines ringing my phone, one GV account straight up forwarding, and two numbers on another GV account, through Hangouts.