To start off (will get more into it later) comcast is rarely ever able to offer customers with the speed they are paying for, for for virtually all users, any additional user using their network will cause a slowdown regardless of what comcast says.
Keep in mind that wifi radios are not 100% efficient in handling multiple users. If your wifi radio can do 100mbit/s on clieent, then if you add an additional client and both want full speed, then both will not get 50mbit/s under a completely fair que. The overhead of dealing with more than one client will cause the sppeeds for both clients to be less than 50mbit/s, you will be more likely to see around 45Mbit/s or less. The only way to avoid this will be to have a completely separate wifi radio and antenna running on a completely different non overlapping channel in roder to prevent the public network from impacting the home user's wifi.
Furthermore the CPU will have to be significantly faster. (as you create more network activity, pingtimes slow down. If you have a program such as ixchariot, you can demonstrate this yourself. Have a rate limited benchmark go between 2 systems, and then have 2 other systems on the same network start their own rate limited transfer, even if you are only using 25% of the available network capacity, ping times will increase as the router is doing more work. On enterprise networking hardware, it is negligable, but for home routers, especially less powerful ones, it becomes more noticable and you can easily add a few extra milliseconds to your ping times if a hotspot user decides to create a constant connection, even if the throughput needed is low, if there are many connections, it will impact the performance of the hom user. Furthermore, traffic shapping will open the door for denial of service. Even on a router such as the WRT1900 AC, that dual core 1.2GHz CPU will struggle to maintain 400mbit/s while using QOS, and from my experience with doing QOS benchmarks with ixchariot on consumer routers, even a low bandwidth datacenter style workload (lots of cmall packets stressin different priority levels, will cause the router to work really hard to manage the traffic, and thus you can get a high CPU load with only a small amount of traffic)
One issue that seems to not be getting enough attention, How many comcast customers are getting the ful upload and download speed that they are paying for? (most do not and in those cases, allowing a hotspot user to get even 1KB/s will mean 1KB/s less that you can use for your home network because they are already unable to provide you with your full speed.
If I were stuck with something like this and was unable to opt out, I would either buy my own modem and not use theirs, or I would take it apart and physically cut the PCB trace going to the wifi amplifier to make sure that the the wifi network cannot be used.
PPS can a comcast customer in the effected area try loggint into one of those hotspots and see if the WAN IP is any different, if not then this means that some people can have fun going to the homes of people they do not like, or choose random homes, and then do things like have personal challenges "how fast can I break every single rule on this side, and get the account banned, and the make many more accounts until they decide to do an IP ban (and can it be done for the most popular websites)
Or if you know that the person loving in the target home is a gamer, then see if they play on any servers that use some form of ban link, and and just wreak havoc.
Overall, this is 100% bad for the customers stuck hosting those hotspots. For all, it will mean a drop in performance (PS if the same wifi radio is used, see if 802.11b is still supported
many routers like to really slow things down and drop QAM levels when a legacy user connects
). For users with ample capacity (to a point wherre they get 100%+ of what they pay for consistently 24/7, then they will only get overhead related slowdowns from the router having to manage more clients and possibly deal with the QOS. For everyone else, on top of th eoverhead, any throughpt that the hotspot user is able to get, will directly slow the home connection because they are in a position where comcast is already overloaded and unable to provide then with the speed they are paying for.