A hierarchy of dumb FIFO queues does not make a real QoS system. It can produce reasonable-looking benchmark numbers when the prioritization rules and benchmark are contrived to match, but in the face of real-world traffic that uses HTTP to carry vastly different kinds of traffic over links that don't have constant bandwidth and latency, OpenBSD is hopeless. Even the rate-limiting capability that OpenBSD has is rudimentary and lacks the ability to account for per-packet overhead and framing overhead, which is necessary for accurate traffic shaping on common service types like ADSL.

If you say you don't see any disadvantages for OpenBSD on QoS, then your idea of QoS is twenty years out of date.