Quadcopter pilots have been using STM32F3 for years. Check out: https://github.com/betaflight/betaflight one of the more popular flight control projects. They've been supporting STM32F4 and STM32F7 for quite some time.
A typical quad flight controller with an STM32F4 and a decent MPU will set you back less than $30.
A typical quad flight controller with an STM32F4 and a decent MPU will set you back less than $30.