The Volt was only "parallel" when running from gas. It was still serial in that when running from battery it only ran from battery, then switched to gas generating electricity, with some mechanical assisted torque in edge cases (usually only past highway speeds or "mountain climbing").
That was mostly because the electrical conversion from a gas generator is still so relatively inefficient and slow compared to a modern battery. The mechanical efficiency of gas engines is relatively better (which is why ICE has survived as a category for so long). Batteries are far more efficient at delivering high power on demand as needed for torque than a gas generator.
Any EREV is going to have that problem and experience those trade offs. It's a unfortunately defining part of the category. It's also why Chevy has said there's no real future in EREV power trains because they are a worst of both worlds situation with too many unfortunate trade offs to consider, such as needing to be parallel in gas-only operation edge cases to make torque requirements.
That was mostly because the electrical conversion from a gas generator is still so relatively inefficient and slow compared to a modern battery. The mechanical efficiency of gas engines is relatively better (which is why ICE has survived as a category for so long). Batteries are far more efficient at delivering high power on demand as needed for torque than a gas generator.
Any EREV is going to have that problem and experience those trade offs. It's a unfortunately defining part of the category. It's also why Chevy has said there's no real future in EREV power trains because they are a worst of both worlds situation with too many unfortunate trade offs to consider, such as needing to be parallel in gas-only operation edge cases to make torque requirements.