Hi Sean,
I have an older B&M taillight powered by an older SONdelux (before the Wide-Body) hub, and it's plenty bright, and a Compass/Rene Herse taillight running from a newer Wide-Body. Also decently bright. I can't imagine SON's own taillights would be anything but reasonably bright.
I too am curious, Mathieu, to know the details of the troubles with dynamo taillights reported by RBW and your LBS have. I wonder if some of the troubles have to do with wiring, which, in the absence of accommodations on or in the frame, must be zip-tied to, handlebar-taped to, or wrapped around the frame to reach the taillight. It's a mild aesthetic irritation, for me at least. Gets the job done, though, and my zip-tie-enabled system has worked every day and in every weather for nine years.
--
Bob