I can well believe it. My XL-FDD is fitted to a Dutchie with a substantial fork and it can stop it from my typical speed very abruptly, easily jack-knifing the trailer if fitted. I suspect it would easily fold my light sports roadster's thinner fork. The smaller X-FDD is probably more than adequate for almost all bikes: as long as you have compatible levers (caliper pull not V, IIRC, but SA branded levers are available) and keep the cable adjusted, you'll lose traction long before the brake is fully applied.
It's not better known because British touring cyclists seem to hate drum brakes.
Spa only sell Sturmey Archer fixed-gear hubs - I don't know if that's due to Spa's reputed quirkiness.
No servicing is without risk (I had a heck of a job with a simple road bike front hub a month or two ago) and detaching/reattaching the connector plug is reportedly a pig.
I believe it will charge powerbanks but I've still not fitted the USB port I bought for mine! I blame this year's main cycling holiday being postponed by some little crisis.
http://www.sturmey-archer.com/en/products/detail/x-fdd has the spec, parts diagram and user manual (the standard version, not the XL).
https://dorkythorpy.blogspot.com/2013/02/sturmey-archer-xl-fdd-dynamo-and-drum.html is one review of the XL.
https://smutpedaller.blogspot.com/2012/02/sturmey-archer-xl-fdd-x-rd3-review.html is another, which I think might be the one
@Vantage saw who eventually managed to bend the forks.
There is also the X-FD which doesn't have the second D for Dynamo on it, and I think you could use the same reaction arm clip on both X-FD and X-FDD (or XL-FD and XL-FDD) but the weight saving is only 450g.
1.22kg for hub, dynamo and brakes is reportedly in the same ballpark as a Shimano hub dynamo plus disc brakes. I see that there's a recently-launched
Shimano roller-brake hub dynamo DH-C6000-2R which is described as "lighter and more compact structure" but it's 960g and I think you have to add
the roller brake itself which doesn't list its weight so I suspect it may weigh more than SA in total!
Plus I bet it's like other Shimano hubs and they won't sell you most of the parts you'd want to service it: if it develops problems or simply wears out, they want you to junk it and buy a new hub. Throwaway society.