GM has been selling mild hybrids since 2007, starting with 36V and then moving to 115V. The additional power & torque of the electric motor doesn't improve acceleration performance, mainly because the peak torque of the electric motor occurs well below the peak torque of the gas engine. In their mild hybrid vehicles, peak power and torque of the combined drivetrain isn't any better than the non-hybrid versions of the same vehicles. But the hybrid systems add weight, so they end up being a little bit slower. The battery capacity is small. It will charge up when braking, and discharge when accelerating from a stop. While cruising, there often isn't much left in the battery to give you a boost. And even if there was, you probably wouldn't feel it. The electric assist you get when rolling off from a stop only makes the engine rev lower. It doesn't have a big enough electric motor or a big enough battery to cruise around the city in silent mode. So the only benefit from the mild hybrid system in those cars was +1 MPG on the highway from a slightly more efficient gas engine and +4 MPG in the city from the regenerative braking and launch assist. Hence they haven't been very popular.
Honda's earlier hybrids were similar, except Honda used smaller, more fuel efficient gas engines in their hybrid models so they obtained a bigger MPG boost but with much less performance. They weren't very popular either.
You probably could design a mild hybrid system to store and save energy for when it's demanded for performance rather than for fuel economy improvement. But that's not why all the manufacturers are interested in them. Mild hybrids are just a cheap and easy way to bump fuel economy by a few MPG. So I think we're going to get more GM style mild hybrids and I'm not excited.