They are extremely talented technicians and so under-rated. Fair enough, they don't take unneccessary chances, probably explains their success over the last few years, but anyone they have fought, even "world champions" such as Chagaev and Peter have been systematically dismantled and made to look like novices, much like I believe either of them would do with David Haye.
Whether or not this is true or whether this is exaggerated I am not sure. But sparring is just sparring, anyone can take their eye off the ball in a spar.