I have just built up a wheel with an Sturmey Archer S2C 2-speed coaster brake hub. Same as the B2C but with an aluminum shell.
Everything worked OK at first. A few clunks when braking, but nothing too odd.
On my second ride, cruising along at about 8-10 mph, I applied the coaster brake. The pedals kicked forward, as with a fixed gear. The wheel continued to spin, snapping the bolt that held the brake arm to the left chainstay. Using my front brake, I was able to stop the bike. I walked it home, not wanting to ride with the brake arm spinning loose.
I replaced the bolt, but the hub is still acting like a fixed gear some times. Additionally, it is ghost-shifting every few minutes. I feel all sorts of strange bumps or hiccups through the pedals. Though it is technically rideable, I would not want to ride it like this at more than 5 mph.
This is a new hub. Chainline is OK. Chain tension was high but not astronomical - about what you'd use for a fixed gear bike. No chain slop.
I never rode it particularly fast, and I am not a heavy rider (11 stone).
I've emailed Sturmey Archer, but have not heard back.
2-speed coaster brake by jimn, on Flickr