It all depends on level of gearing.

The 4x warlock & 1x priest, who doesn't do that well against the DK/Ret team... how are their gearing in comparison to each other? Are they both 1000+ resilience, or is one team in PvE gear? From the comment that a single warrior can Bladestorm the team down, I'd guess they're using PvE gear or are very undergeared.

Fear is subject to diminishing returns, and you'll have at least one set of trinkets against it. But they'll have five round-round instant cast fears (assuming Affliction spec). If they go Destruction spec, then they have four Shadowfury casts, also on round-robin. The priest can mass dispel bubbles away.

Still cloth is rather vulnerable to physical damage, and the DK/Rets will be putting that out in large amounts.

There is the comparison of a boxed 5-man team vs a different boxed 5-man team, and for arena that is probably the more important comparison. But for battlegrounds, it becomes more along the lines of a random group size vs another random group size. In theory, we're only concerned with larger groups of opposition, because if we're not at least competitive against large groups then a given composition won't go far.