Does your friend you are mentioning may share the references of his claims?
MPTCP has no issues with head-of-line blocking, because the receive-window is not on a per-subflow level but rather on the data-level [1].
I don't see, how MPTCP could cause extreme congestion, because the coupled congestion control makes sure that MPTCP is fair to other flows across shared bottlenecks [2].