My concerns:
- Security. We all know how IT security works these days - it's secure until it's not.
- Speed. They say it won't impact your speed, but we all know how cable internet works and I'm sure the routers have limitations (RAM and CPU power). If you have several free wifi users connected to your modem doing moderately intensive stuff, I'd be interested in seeing the results of a speedtest on your private network.
- Legal Issues. If someone is browsing unsavory/illegal stuff from your router, it's going to come back to your IP. Other people have to login with their Comcast info, but you're depending on Comcast to save your ass with logs.
- People. People can be insufferable. I can see disputes happening over a crappy signal, the homeowner unplugging their router, free wifi users loitering, etc.