Arris SB 8200 and DOCSIS 3.1

I'm using the Motorla SB 6121 modem.  Xfinity has been sending me messages that my modem is not capable of the speed that I'm paying for and that I should upgrade to a DOCSIS 3.1 modem. So I purchased the Arris SB8200.  I couldn't get it to activate.  Called Xfinity support -- they couldn't activate it.  A technician came to my home and couldn't activate it -- said it must be faulty.   Sent it back for a refund and am back to using my SB 6121 which works fine, so I'll just stick with it for now.

However, I'm reading so many negative reviews about the SB8200 and really wonder what is going on with this modem.  Are there really so many defective SB8200s out there?  Hard to image.  Seems more like some kind of compatablilty issue with  it and the broadband cable signals by providers. I've been in touch with both Arris and Comcast about this issue and am really surprised that neither really has supplied answers to what the issues are?  The tech that visited my home said my signal is strong enough.  I don't have Gigabit service but this seems to me to be an experience that will disuade me from subscribing to Xfinity gigabit.   Century Link offers fiber in my neigborhood and Google fiber may arrive soon,  so I have those options.

Does anyone have answers as to why so many hare experiencing issues with the SB 8200?