if he only paid 100$ for a modem and it was brand new, it likely wasn't an all-in-one station like what large ISPs are trying to push. Cheaper/dumber modems severely limit the amount of control Comcast have and how much they can fuck up your network with a bad update. They can't mess up your wifi settings if the box they control doesn't even have a wireless radio on it.

"dumb" modems are a lot more reliable simply because there is nothing for them to patch inside. It doesn't have a complex OS running a wide range of services that need regular updates (managing a TV, wifi, file sharing, etc.).