SFC doesn't have a problem with Cable / DSL itself, actually, SFC enjoys the higher speeds (less lag etc...). There are many people in this community that connect with either one of the 2 methods.
SFC does have problems with some of the associated technologies that work with high-speed connections. SFC seems shaky when played through a software firewall, and cannot play through a router / hardware firewall without "significant" work, ie, forwarding certain ports to the gaming PC or putting that computer on the "DMZ", which effectively opens all the ports.
Myself, I have a cable modem, and a router. It wasn't intuitive, but I managed to get the DMZ feature working on my setup, and so far, I've only ran into "connection issues" with 32nd_Shin. There are enough of us out here that would be more than willing to do what we can to help you if you run into problems, just time the problems to occur during a test server for best results...
(ie, no frustration from not playing on the main game, but enough techies around to post...)