I have been on comcast and have been using tivos with cablecards for around 7 years now and I have had all sorts of problems but eventually get them solved. Here are my thoughts.
1) cablecards are a nightmare. Before you install the Roamio, show up to a comcast service center and pickup like 3 of them. Yes 3 of them. Just return the other 2 you don't end up using later.
2) If using wired ethernet for the Roamio, DO NOT put in your wireless settings even to test. I had horrible issues when I added both wireless and ethernet settings where even when I wanted to use ethernet, and it showed connected as ethernet, it was still connecting to wireless and causing MRV and streaming issues.
3) Setup your tivo subscription for the Roamio before turning it on. I guess transferring a plan from another tivo can be a hassle so when possible, just create a new plan for the roamio and if it's replacing a box, just cancel that old box's plan after the new roamio is setup and going.
4) before installing cable cards, connect to internet and run guided setup and have the major update roll out. Tivos take a day or two to get fully 100% operational. When I mean 100% I am talking about every last feature. Annoying but you deal with it.
5) Call this number and pray you get a good tech - 1-877-405-2298. Don't ever send a tech to your house. Don't spend hours on the phone. A successful cablecard install takes 5 minutes tops. Try activating the first card and wait for them to send a signal. It should work pretty quickly even though they might tell you it takes 30 minutes. If it doesn't work, don't try and troubleshoot it but move on to the 2nd cablecard. And then the 3rd. If all 3 don't work then the easiest thing and what has worked for me is take a working cablecard from a tivo I am replacing and have them reactivate that card. If all 3 new cards won't activate and they can't re-activate a card from a working tivo, the tech you are working with is an idiot and you will just have to request a new tech.
6) if you get some channels but not all of them or no premium channels, the card needs to be re-activated or the tech did not pair it correctly. You can tell them to delete the card out of the system and re-add it as a new one.
7) for the first 48 hours don't freak out if your other tivos don't show up in the Now Playing list or if there are bugs. Force a bunch of service calls, reboot it a few times, make sure the updates are all rolled out and then after 48 hours call Tivo or check these forums if you are still having issues with anything.
I'm sure a lot of people will say these steps are overkill and they get their tivos up and going 100% in an hour or two, but they are not the norm. When I am realistic and plan for 48 hours of a setup and work in period then I usually end up really happy with my Tivo. I love college football and I had 6 HD games recording at once on Saturday while my wife was streaming to ipad. The Roamio is a great device once it is all finally setup.