I am thinking about taking the jump and replacing my cable modem with a simple modem that I can buy. My trepidation is that once I do this, if I have an issue, Cox will blame the modem. Are my fears justified?
And what if they update the connect to symmetrical up/down. Can I then take advantage of that with my current modem?
Well as a sample of one, I advised a friend on which modem to buy when he was moving into his first place in one west coast state, and he relocated to an east coast state and is still using the same modem without any issues. He bought an Arris Surfboard DOCSIS 3 modem. It was more than he needed at the time for the speeds then available, but it was intended to be somewhat future proof. These days, there are DOCSIS 3.1 modems that supposed to be the most future proof.
There are two things to consider: whether it will save you money and whether it will actually be optimal enough.
Check first that any modem you would buy is supported by your ISP. The modem probably has to be configured specifically for the ISP, so even though you will own the hardware, they will take all control of its configuration. They may even reload the firmware to be compatible with their network. This does not guarantee the perfect or optimal configuration. They might have optimized their network for a specific modem, with sub-optimal configurations permitted for a list of other equipment. Odds are you cannot buy what they optimized for. Whether this actually matters is based on the number of channels they use and the number you need to obtain the level of service you’re subscribed to.
Remembering it’s a game they’re controlling and you’re just playing, you may find a situation like: they offer 250 Mbps service. If you use their equipment (which you need to pay to rent) magically you mostly get the advertised rate. If you use your own equipment, mostly you get a lower rate, say 200 Mbps, and they quote that their plan says “up to 250 Mbps, with no guarantee… blah blah.” There comes a point where fast enough is enough, and saving that $10/mo is more important that the missing 50 Mbps and a constant fight with your ISP.
Granted, in my sample of one, this never happened, but it could have… and I think you have to be okay with that possibility if you’re gonna go that route.
Thanks. I will take the jump some day. My biggest concerns is that I get an issue and cannot do work and/or the cable company blames my router and ends up charging me a $75 service call and I have no way to know if it is really my modem or their system.
Any reason you can’t swap the old Cox modem back in if you ever need to raise a fault? Or do you have to send it back?
Here the ISP hub is included in the broadband price, and they don’t seem very motivated to get them back if you switch ISP or use your own. I have quite a few in a box somewhere!
They should have a list of approved modems. Yes it’s true, you could get something faulty, and since you own it you would be responsible. That’s inherent with the risk of owning the equipment. But odds are if ti would fail, it would fail within the first month when you could return it, or maybe within the warranty period.
I would certainly shoot for the highest DOCSIS on the list that works for your budget, and if you want the latest and greatest fastest speeds when they become available. A lot depends on your area and how much competition there is, but here there are rumblings of Spectrum getting symmetrical down/up speeds, which I think is DOCSIS 4, soon. And while our speeds were getting bumped for free, you needed a more compatible modem to take advantage. Still ditched Spectrum for T-Mobile Home Internet though 
I think once you switch it out, you call them and they activate your own modem and deactivate theirs. If I do that, I add more risk to something not working if I switch it back.