I didn't have to file a FCC complaint with Xfinity but I had an issue with Arris/Moto surfboard modems and a miswritten boot file some years ago that took weeks to fix. Basically, you can have your own equipment on a dynamic IP'd business account but they automatically default to calling you the asshole for any problems that get reported.
Business class had launched new speed tiers and it broke upstream bonding on my modem, of course they blamed me. So I bought 2 more modems (same model and a different one) and the same things happened as soon as they got provisioned on the account (prior to this on Comcast's provisioning walled garden they'd show the upstream as bonded, but once activated on the account upstream bonding would turn off and speeds went to shit). By this point I had records and emails with like 4 or 5 different people, a useful thread on dslreports, and a very worthless thread on reddit.
I took all this info and CC'd everyone I spoke with in an email to the CEO of Comcast. Executive response had someone up in Massachusetts call me the next day confirming the issue was with their provisioning boot files, and they credited me for like 3 or 4 months of service. Overall I was pissed it was this difficult to escalate but at least once it was in the right office they handled it. It probably helped that this was a business class (which overpays for better support vs residential script readers).