Apparently regenerative breaking efficiency in bikes is rather limited (small motors / generators, high friction). It still increases the range a fair bit (enough to be a better investment than bigger batteries), but efficiency is still not as high in bikes as in bigger vehicles which can drive more kinetic energy into bigger generators with better individual wheel control

Some paper says ~25% extra range in bikes at the high end vs ~50% energy savings in Japanese trains. Different units for those numbers, but you can infer that trains has much more efficient regenerative breaking because that number indicate a doubled range for the same amount of energy used.