My big issues with rear bike lights are:
(1) They're not bright enough or don't give enough coverage
(2) They don't have enough run time
(3) They usually don't have user replacable batteries
(4) The ones with external batteries use flimsy wiring
(5) Totally waterproof units are hard to come by
(6) Most don't use shaped beams or claim they do but don't really
No chance anything currently on the market except for generator run lights is suitable for touring or all day cycling unless using the most feeble settings - which pretty much defeats the whole point. Finding a light that addresses ALL of those has been a real challenge.
The closest I've come so far is adapting some automotive 12V LED lights to mount on a bicycle and run off 11.1V RC Lipo batteries. About 15 or 16 hours of run time for dual lights on solid with a good combined 500 lumen shaped ouput that doesn't dazzle following traffic but paints the whole road red behind the bike - clearly visible from 2000+ feet