The custom/easy mixer in PX4 is both a strength, and a massive weakness. It makes it quick and easy to get new vehicle types flying. But it also means that PX4’s flight controls are nothing more than really simple PID loops. This is highly over-simplified. This is why you will usually only see PX4 flight video in a laboratory, or on a windless days, and at low speeds, etc. Generally low performance flight.
Ardupilot has extremely sophisticated control systems. This prevents using flexible mixers. But, results in high performance flight.
Here’s a clear example of the relative performance. Here’s the state of the art flight performance for PX4 on a helicopter:
And here’s what Ardupilot is doing:
Basically, PX4 gets you little more than a basic UAV operating system. Ardupilot, is ready to go to work. Just that simple.