Comcast supplies any new customer with a D3.0 modem from one of their suppliers (Arris, Cisco (now Technicolor) or Technicolor). Some of these models are known to have horrendous wifi. If you get one of the old models, Please please turn to "BRIDGED" mode and buy a decent dual-band 802.11ac router.

OR

go ahead and purchase one of the retail modems and a seperate. If you want option of upgrading the Firmware and controlling your Router, buy a separate modem and separate Router. A customer CANNOT upgrade any Firmware on the modem or if is a Combo device. If one of the Favourite vendor has a bug with Portforwarding or something, your modem is pretty much paperweight unless Vendor is willing to fix it quickly and Comcast can push the FW to you quickly. All this can easily take 3+ months for Urgent bugs.

If you are buying new modems and are on service tier > 100 Mbps, Please buy 16x4 Bonding Modems as minimum

If you are on extreme 250 or 300 Mbps package, Please buy atleast 24x8 or 32x8 modem. 32x8 modems based off INTEL puma6 had a critical latency bug but I have heard it is fixed in the field now.

If you are on Gigabit, you kind of stuck with Comcast modem for now until they let people active DOCSIS 3.1 modems. They can activate for other tiers but NOT gigabit.

edit: grammar