How is their overall hardware support?

If nothing else the vendor-specific crappy toolchains do support the vendor’s HW well, so that’s something they need to do so that devs don’t have to start their projects with implementing the low-level support by themself.

Very good? Zephyr generally wraps the hardware SDKs for the things built into the microcontroller. And then their own portable drivers for any outside peripherals, like I2C/SPI/Onewire/etc.