DF content is able to be completed with any party comp. The best party comp doesn't mean jack if the players themselves aren't that good. All things being equal, a party like 4 bards is going to kill things a bit slower than an optimal party comp, but it's still doable. Does it suck? Yea I guess, the DF should be able to better match party comps (I do think for the most part it does a good job of this). I don't recall ever having as strange of a party set up as that, but it's random. I don't think it's a "double standard". Classes with more utility have less throughput. See Paladin, Ninja, Bard, Machinist, and Astrologian. It's not specific to brd/mch.