Were these "strict policies" licensing issues? Or coding standard issues? IMO these issues are orthogonal to ease of integration.
Writing a simple driver for say an I2C sensor is quite doable.
However, getting a driver into mainline requires conformity both in terms of licensing and in terms of coding standards for the whole library, including 3rd party code.
I contributed to port a new microcontroller, which includes writing device (peripheral) drivers.
It was pretty much about their coding standards. I had that experience because I was contributing for mainline branch. They think avoiding abstracted manufacturer peripheral libraries (like STM32 HAL) is the best way to keep the binary small. It is pretty much appose to what Zephyr OS is doing.
I think peripheral libraries are adopting BSD license these days so the license shouldn't be an issue these days.
NuttX also has strict licensing and coding standard enforcement. Such things are a necessary component of QA and I would think that all serious RTOS's have similar standards.
Writing a simple driver for say an I2C sensor is quite doable. However, getting a driver into mainline requires conformity both in terms of licensing and in terms of coding standards for the whole library, including 3rd party code.