Comcast tends to make it hard to bring your own modem. The list they give of the ones that will work is frequently out of date, once you have one you may or may not need to talk with them to get it recognized, and they may just lie to you instead of helping.
Beyond that, its just that, again, they don't tell you when they turn it on (until months later, I eventually got a letter, though I already knew they had). I had tried to turn it off via the account settings (there's a button to do so), but it always has "an unexpected error". I eventually managed to get it turned off by talking to an agent (I wasn't in the mood to fight with a third party one that should work but doesn't because Comcast's a dick company).
Now, even after all that, it still doesn't matter: I live in a very high density area, and there's something stupid like 30+ xfinity wifi hotspot within range. The 2.4ghz band is totally useless from interference.
Easy right? I just got a 5ghz router, and that works great. Oh, but the Wii U is 2.4ghz... Well, I'll wire that. My TV is 5ghz so thats fine. My roku is 2.4ghz...I could buy a new one. Oh but the Nest thermostat....and the list goes on.
Its just something they shouldn't be doing, flat and simple.