Rivian's TM-B electric bike

808Wh battery and 100 miles of range. These two numbers track with each other and are roughly believable.

OTOH, with a battery this big, a generator powered by the pedals, and regen braking this thing has to be heavy. I'd expect it to weigh at least 80 lbs. More likely 100. The fact that their "specs" say nothing about weight suggests they're embarrassed about the weight.

Regen braking is how you can tell this was designed by a moron. The energy balance simply does not favor regenerative braking on a bicycle, especially a bicycle that flippantly ignores aerodynamics like this one does. A bicyclist loses roughly all of their energy to air resistance. It's not a truck. There is not substantial potential energy to be recaptured going down hills.

What do you mean? The regenerative braking only kicks in when you engage the brake lever. It's not going to add much range but it's free, I don't see any downside to including regenerative braking.

You're going to have a difficult time pulling a lot of energy out of the back wheel as you're slowing down. The more you decelerate the less weight you've got on your back wheel. Eventually you reach the maximum energy transfer from back wheel contact patch into the motor and lock up the back wheel, and even then you may not have considerably slowed the bike.

Regen on the front wheel would be most effective - but then you've got two motors or a less-than-ideal front motor that adds unsprung weight and has similar traction issues during acceleration as the front unloads.

It's a shame - I think a lot of people want ebikes to work, but they're not as convenient as a pedal bike (especially not in small apartments) and usually they're too heavy to really use in blended pedaling/e-assist mode.