Hello,
The reason I use it is because I can set the router with the functionality I require, without additional junk that I don’t need. And I don’t like the interface they force on you.
As for PCI-e vs usb, you theoretically shouldn’t see a difference at least until 5Gbps… so it doesn’t really matter that much what mode you are using. Besides as you said - on a good day. The speed depends a lot more on the provider and environmental factors unfortunately.
Of course if the vendor provided firmware works for what you need, then good for you.
So… what you use depends a lot on your use case… and what you like.
Zhivko