All depends on what one you get, in this case it was not a problem of restriction but of losing connection. Some combo's supposedly are very nice if you get gigabit internet it takes a decent modem/router to handle it.
In my case it was very restrictive on combo though. It stopped some ports I used at one time when using sites that I could allow people to rent my hash, also part of my security system. What I did and others can to is call you dls/cable company and tell them of problems and say you want a passthrough mode on modem. If they are a good ISP they will work with you and you can use your own Router.
That is what I did and ended up getting a blackhawk router which has done pretty darn well overall. So I'm happy with my gigabit internet on my own modem. But did take ISP to enable passthrough to allow this.