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If the battery is depleted, and the electric motor cannot be used on slow speeds or up hills to assist, then the ICE that is designed for optimal higher speed cruising at a peak efficiency - would not have enough torque to accelerate from a stop, and/or go up a hill and/or into a headwind without a multigear transmission.
What this means is, a parallel hybrid by definition, would need to be able to move the vehicle with either the ICE or the electric motor - which changes the requirements for the ICE; making it bigger than ideal - which adds to the weight and the complexity of the design. More cooling is required, and the mechanical drivetrain becomes larger.
It also means that the control systems or the driver has to do the mix-and-match of what propulsion is used.
All this is for a "pure" parallel hybrid. Most hybrids today are multi-mode in that they can be serial or parallel or both at the same time.
A "pure" serial hybrid on the other hand, has no need to have a larger than ideal ICE. A serial hybrid is essentially an EV with a genset, so the electric motor is the only one that has to be sized to move the vehicle, and the load on the ICE is known and fixed at all times. The ICE only runs periodically, so the cooling system can be closed off when not in use, and the ICE will warm up more quickly because it is a smaller displacement.
A serial hybrid will not run out of battery charge, so the "rhythm" of the drive and load profile is not dictated by the drivetrain. It is much more flexible, and the design parameters are much more tightly defined.
The FVT eVaro is a serial hybrid and Ken Fry's 'Zing' is a serial hybrid. There are no pure parallel hybrids, as far as I know. The "closest" is the XL1. Most hybrids are mixed mode (Prius, Volt, etc.) and some are so-called mild hybrids, with the electric motor limited to boosting the peak torque.
The XL1 demonstrates the only way I think a parallel hybrid can be done. It can only put energy into the battery with the plug, or with regenerative braking (which is only on the brake pedal, incidentally) and it can do charging only when the driver holds their foot lightly on the accelerator pedal.
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