Not. I'm sure there are modifications that can be made to improve them (and I'm sure Geoff Apps can tell you far more about that than I can) but stock Sturmey drum brakes aren't good stoppers. They're mediocre in all weathers with zero maintenance though, which is their advantage.
The above is basically correct and Geoff Apps has more experience of using drum brakes off-road than anyone.
I really meant the brakes. But for off-road use, this doesn't seem to be a great compromise - you add a hell of a lot of weight and end up with a narrow gear range, limited tyre choice and mediocre brakes. For an errand bike it's a different matter, but it would make more sense to start with a frame (Post Office surplus?) that's built to take the wheels.
As I have been riding mountain bikes fitted with drum brakes since 1984 and last year built a hub geared/braked bike, here's my take on the subject.
. Most hub brakes have a fundamental flaw in that when new, they work well. However the two brake shoes wear unevenly. Eventually only one shoe touches the drum and so the braking effect halves. Try this link for a more detailed explanation.
Both hydraulic hub brakes, Shimano Inter8 Roller-brakes, Highpath hub-brakes, and the brakes Geoff Apps used on the Clelands, do not have this problem.
Hub brakes weigh more than disk or rim-brakes.
If badly designed, they can make the wheels more difficult to remove.
They are usually very low maintainance. One set of Highpath brake shoes lasted more than 20 years of heavy off-road use.
Good hub brakes have superb modulation characteristics and so provide unrivaled control. They deliver the braking forces much more subtle, gradual and progressive manner than any other brakes, and so are much less likely to lock up unexpectedly. This can be described as an anti-locking characteristic, though if you pull the levers too far they will eventually lock up.
(On a recent 1 in 3 down hill in the snow, the riders were surprised that I could control the decent without locking up the wheels. The others had to descend with their rear wheels locked).
The king of the hub brakes at the moment are the Shimano Inter8 Rollerbrakes.
They use a system of cams and grease covered rollers and are silent and smooth in operation. Most other braking systems fail if contaminated with grease!
The downside is that they are heavy and though lightweight versions could be made this is unlikely as the Dutch and Belgian market they are intended for are not very weight conscious.