MediaTek chips are well supported by OpenWrt. Broadcom is not good supported. Mainline Linux kernel supports recent MediaTek Wifi chips quite well [1]. MediaTek is also working on these upstream Linux drivers, but they still have a proprietary Linux driver in addition.

Also the rest of the recent MediaTek SoC is supported quite well by upstream Linux and OpenWrt.

You can run OpenWrt on recent MediaTek SoCs with all code running on the main CPU being open source, no closed source code needed inside the Linux kernel address space or in user space. The chips need firmware running directly on the IP cores. It needs a firmware running on the wifi core itself, there are probably one or more CPUs inside the wifi cores doing real time stuff. The Ethernet PHYs also need a firmware which is running on the PHY.

[1]: https://elixir.bootlin.com/linux/v6.17-rc5/source/drivers/ne...