Apparently, MY impression was just the opposite -- I thought the Campagnolo shifters were more efficient AND reliable when downshifting under load ...
I NEVER experienced a balky shift onto a larger cog with the Campagnolo shifters ... and, I am downshifting on an 11-32 (sometimes, 12-34) XT/XTR cassette. The first time I realized that the downshifting was so effortless I knew I could go back to a double ... okay, a double with a MTB cassette.
Nonetheless, I do recall prior occasions when the Ultegra 6500 shifters did not engage quickly to the point of balking on what I would consider intermediate inclines ... so, I ended up putting a triple on my bike so I could dump out to the granny if I needed a lower gear.
I suppose that with a 12-27 cassette, or smaller, it might not make a difference engaging the larger cogs.
AGAIN, your experiences & impressions may vary ...
BTW. I'm riding on mountain roads east of Albuquerque where the elevation begins at 6500 feet and goes up from there ... the flats are "false flats" and the inclines vary.