Benefit of using px4 for simple servo control

Why should i not control the servo functions like payload, radio communication etc from onboard computer? Is their any disadvantage of controlling servo/payload mechanism from on board computer instead of flight controller?

The onboard computer is more complex and more complicated system then flight controller (FC), and FC is more complicated then PX4io. It means that complex systems have higher probability to brake.
If you have few kW motors and your onboard computer will restart for few seconds (minutes), in this case few kW of energy will be uncontrolled. The ESCs and servos have no failsafe mechanism inside.
That’s why the motors/servos control divided into the independent module that has own CPU and making simple work to detect the failsafe situations.