850L is an insane amount of light. For the average rider, that's going to be nearly 3 times the amount necessary to see at night, if the light is focused properly. I started out with a Light & Motion Solo 13W halogen system that pumps out about 290 - 310 lumens. It's not the most awesome light, and I feel like I'm starting to outrun it at 20mph, so it's not great for midnight downhills; but it works fine for visibility of rocks and potholes and critters darting in front of your wheel in the wee hours of the morning.
The reason why an 850L light might not seem bright enough is because very few companies selling high-powered battery lamps are using focused optics. This means that more than half of the projected light is being wasted. It's lighting up the sky or objects way off to the side of the road where you just don't need to be concerned. It's great if you're doing a 24hr MTB race and you need to watch for overhanging branches on a trail, but if you're riding on a road or MUP then it's not doing you any good.
Lumens are a non-directional measure of light intensity. Most generator lamps measure intensity in lux, which is a measure of intensity over a specific area (1m^2 at 10m distance). So, while the lumens may be lower on a generator lamp (usually ~400L), the lux rating is up in the 80 - 90 range, and a symmetrical high powered lamp might crank out 800L but have a lux rating in the same range because the intensity drops pretty sharply outside of the center spot of the focal point. This is how the lower-power of a generator hub manages to put out enough light for good visibility at a very long throw distance.

I have a Princeton Tec Quad LED hiking light on my helmet. I use it for spotlighting street signs in the dark while randonneuring, reading cue sheets/maps, and fixing flats. There's just no way to use a generator lamp for any of that.