This pretty much spells this out, and since most bike lights and flashlights tend to have small hotspots for distance and very limited spread, they're great for attracting attention, less effective at lighting the complete lane up efficiently, and not too likely to nail motorists unless deliberately pointed at them. I'd say the biggest place they might be a danger for oncoming traffic is on a narrow bicycle path where they're least likely to be used at full power.

There's a reasonable limit to everything, but again, its normally determined by lux and not just lumens. 200 isn't worth worrying about and 2,000 would only be an issue for motorists if it was coming from a major flood-light source - which would definately encroach on their lane even up close.