OK, this isn't exactly a SMC topic, but then again, what is?

I am installing a faucet for a water filter and need to drill a hole in the sink. It's porcelain coated, cast-iron. The directions for the faucet state about 5 times NOT to drill through a porcelain sink. I have, however, found tons of info about doing it and no real problems. Everyone says to use a diamond hole saw to get through the porcelain and then a bi-metal hole saw to get through the metal. I can't for the life of me figure out why the faucet instructions are so adamant about not drilling porcelain. Has anyone done it? Any "gotchas" to look out for?

Thanks!
Gary