In all honesty, I truly believe that if you can get small smooth stoned gravel, it is perfectly fine for them, however the stones do need to be rather small (small enough for them to shuffle around and dig through for scraps). In what I have experienced thus far in my tank, and I actually found a lot of reading that supports this, is that elevated levels of nitrates leads to barbel erosion in Corys. I have smooth stones in my tank now that could be smaller, and everything was fine for months -- my nitrates were also usually no more than 10ppm.]
I got some Plecs and started feed algae wafers, and the whole tank tore them up. Within probably 3 days, my nitrates were up around 40ppm+, and that is when I noticed some of my Corys started losing barbels. Two lost most of them (they also died subsequent to an Ich outbreak), and the thirds are now growing back now that I have my nitrates back under control.
All that said, obviously rough gravel is bad because of the sharp edges, but I think one OTHER drawback of gravel in general is it's ability to conceal waste. I have about 2" of gravel which in hindsight was bad, and when I vacuumed around where I dropped the two algae wafers, I pulled up SOO much crap it was unbelieveable.
Moral of the story --- sand is preferable on two accounts -- first, it is smooth and they can root around in it for food and not damage their barbels, but a close second is that almost all waste stays on the top, so when you dont have residual bits of detritus working their way to the bottom of the gravel bed and polluting your water quality.
