I'd suggest that for Roku to be competitive they will need a reliable way to limit bandwidth. I have been looking at switching to Roku for several years now and haven't done so because of bandwidth. I need to be able to stream via a cellular network and a bit rate feature would allow me to do so with Roku
There is a hidden menu on Roku devices that has such a setting. However, since most online providers use a variable bitrate streaming method, the Roku option doesn't really work on most sites. With Netflix, you can only subscribe to their content at SD levels. I don't think many others have such an option.
Even if the hidden menu option was successful, Roku states that they can remove any and all such settings at any time. So it's not something you can rely on.
There are some routers available that offer bitrate control for Internet access. I suggest you do some research in that area. I don't know of any streaming player that offers such a control function.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.