OPD vs driving should technically yield the same results within a negligible delta. That said, this also depends on the car, the calibration, how the brake pedal "by wire" is programmed, etc.
Example 1:
Audi & Porsche tend to favor "coasting" over one pedal driving. This allows for you to roll without pressing any pedals. They feel this is more efficient, and also more natural. That said, the only way to get MAX Regen on the J1 Platform (Taycan, eTron GT) is via the brake pedal, which is both regen AND friction brakes, on the same pedal. So you have no OPD option.
Example 2:
Other day I was watching a video, forget what new EV it was, but basically they had designed it to only use the brake pedal for the friction. If you wanted Regen you had to turn it on, and that gave you varying levels. In that instance, they opted to ensure the brake pedal felt like a conventional car.
Example 3:
If you are not using OPD, you're still at times essentially doing the same thing as OPD. Most of the time we're driving down the road we're using minimal power, and in cases where you need to move your foot, there is a moment where you may be coasting before braking, and the "braking" perhaps is just Regen, but the argument is that you're doing the same thing just with 2 pedals versus one. This is why Tesla, Volvo, Volkswagen and others hare given you the ability to turn this feature on. Matter of fact, some arguments have been made that this is more efficient because you can more granularly control the power-to-regen more swiftly, quicker, with a single input point.
FWIW ... one-pedal-driving is universally loved at our house (myself and my wife). I actually hate when I have to drive a car w/o OPD.