I'm not really convinced that lifecycle carbon emissions are the right measurement for most people in the US (and maybe other developed countries, idk). A car is already a given for most people and is a possible option for every trip you take so the best bike is the bike that replaces the most car trips.
A steel frame bike might release less lifetime ghg than an aluminum frame cargo ebike but it doesn't take that long for the ebike to catch up if the limitations of the steel frame means choosing the car more often.
napkin math:
(ebike_mfg_ghg - steel_mfg_ghg)/(car_ghg_per_mi - ebike_ghg_per_mi) https://www.wolframalpha.com/input?i=%28320kg-35kg%29%2F%28400g%2Fmi-%2841kg%2F15000km%29%29
An aluminum ebike only has to replace 720 miles of car trips that the steel frame wouldn't to cancel out it's additional manufacturing emissions. In my experience that really not a crazy number when ebikes:
- expand your radius of effective trips
- reduce the time it takes to make trips
- expands the number of destinations (don't want to be too sweaty or tired)
- can safely carry much more (grocery or shopping trips)
In the long term with changes to infrastructure or density (or if you already live in NYC or similar) then I think lifetime emissions of bikes starts to become a more effective measure to watch.
Iirc the biggest reason they can be dangerous is that the dropouts (where the rear wheel mounts) were never designed to stand up to the torque of a hub motor. You can brace it with extra metal or stick with a low wattage motor and make them pretty safe.
https://www.ebikeschool.com/torque-arm-need-one/