They make a miserable mess in regards to salt creep. As far as bad, yes they can cause problems with fish and sessile invertebrates. Gas bubble disease in fish and with certain sessile inverts will clog tissues causing necrosis.
The main idea behind airstones is aeration but this is actually the least efficient way to accomplish it. As long as the water surface is churning, surface gas exchange is occuring. A sump return, powerhead or canister filter will accomplish the goal on a much larger scale with less problems.
Cheers
Steve