> some experts support different priority classes for different application types

That's a thoroughly outdated QoS paradigm. It's almost impossible to make rule-based traffic classification neutral and fair, because it always privileges existing applications and protocols over upstarts that aren't correctly identified by the ruleset. It's also far too easy for ISPs to "forget" to properly classify their competitor's traffic when the ISP is also a content provider, and poorly-designed rulesets can be gamed by using protocols/applications on non-standard port numbers.

Fortunately, this QoS paradigm is no longer necessary and the state of the art for QoS has moved on to techniques that only need to look at the quantity and size of packets in order to correctly infer the correct latency vs throughput tradeoffs for each traffic flow. Packet scheduling is now much more like CPU scheduling, in that it works well enough out of the box without requiring manual tuning or prioritization.