TCP congestion control is a disaster. Setting DCTCP slightly improves things but for big slow pipes it can be impossible to get near line rate. Congestion control's intent also differs from QoS/CoS. It's also a 'reactive' design rather than a constructive one, usually a bad sign. Not to mention that TCP is itself really complex and susceptible to receiver resource exhaustion.

Still getting into the reading here, but making my intuition a little bit more explicit, it seems that at least the design, but possibly the implementation of Homa actually hinges on datacenter switches QoS queues being configured with a policy to explicitly respect the receiver-assigned priorities... cool!