> how difficult it is to build a frame around [butterfly bars] without compromising the handling
The issue maybe relates to the way these bars bring the hand position so much farther back than usual... to me, this just means putting a longer stem on to compensate, it seemed to work fine in combination with the (rather long) Softride stem on my old Bruce Gordon. And, as I discovered with Bruce Gordon (the man), even professional bike makers can have their own rather idiosyncratic opinions about bike fit and suchlike, so I take just about everything anybody says these days with a pinch of salt, no matter how "professional" the reputation. I've had crappy advice from very esteemed people... Problem is, without your own experience to use as a measuring stick, you can just never tell when someone is giving you real gold in terms of advice, or when it's just their own prejudices and personal preferences coming out.
I'm not saying Roberts is full of shit, far be it from me to even think such a thing as I have never talked to the guy. But I have used butterfly (trekking) bars on a couple of bikes now, and the handling was fine. Maybe he has a certain way of building bikes that he's used to, and these bars throw out the little calculations that he's used to making - I can see that would be a pain.
Oh, I can see I'm gonna get flamed for this one. Just to be clear - I'm not making any aspersion against Roberts. For all I know, he might be completely right... but there are a lot of bikes out there using these bars.