Braking is not brake pressure limited, it's grip (tyre) limited. that's why we need ABS. Only having brakes in the front means you give up around 25 to 35% of braking ability (it's not 50% because if load transfer etc) Your front tires will lock up while the rear will have unutilized grip. I dunno how the pressure profile works with Regen, but making it match the characteristic of physical brake pads might not be so simple, the linearity, engagement speed etc will need to match.