I've always had my own modem and I've never had a problem with Comcast/Xfinity not helping me. Maybe I'm the exception.

Recently they updated my area to DOCSIS 3 (or 3.1, not sure). My DOCSIS 3.1 Motorola modem (found this out later) has a firmware bug that causes random modem lockups. Comcast went as far as replacing the cable from the telephone pole to my house in the middle of winter (it was ridiculously cold that day), running signal analyzers, and various other things before looking into the modem.

Also, Motorola Modem customer service was shockingly good as well. They sent me a free replacement modem while they worked on the firmware updates. Provisioning that temp modem was as simple as using the online provisioning.

Now, it would be nice if I didn't have to set annual calendar reminders to call them for when they increase my monthly price by 40%...