Amateur conjecture: Inbound QoS is worse than pointless and reduces performance.

If you assume your bottleneck is your WAN connection and everything on your internal network can handle all data inbound, then applying QoS at the router won't change what is sent over the WAN, only what will then be dropped. The bandwidth has already been used and so you are throwing away the data needlessly.

Corollary: Out-bound QoS is sensible and should always be used.

WAN bandwidth is the scarce resource and has to be managed. You want to prioritise what goes over it. You can push all you like but the router can only send out what you've set on QoS.

Am I completely wrong?