For example, as you stand on a bike stopped at an intersection you have your foot resting on the petal, ready to take off. As you launch, you put a lot of force on that petal and rely on the feeling of connection to the ground to get going.
This motor is going to have to hold that force statically and have a control system with sufficient power and bandwidth to emulate the familiar feeling of the ground. Not impossible I guess, but I wonder how well it works.
Regarding efficiency, I think this is a smaller issue than it seems. Let's say a chain is 95% efficient and their system is 85% efficient (they claim 5% less efficient than a chain, apparently, but I'm sure that's a stretch). Most energy cycling, if not grinding up a steep hill, is dumped into air resistance. But speed only goes up with the cube root of power with regard to air resistance. This means (given the hypothetical numbers above) that you'd go 3.8% faster with a physical chain (or 1.7% if you believe their loss numbers). That difference is not perceptible on a bike and not an issue for 'getting around' use cases.