Salt is not harmless, absolutely. It's just harmless compared to your other options. By the way, all forms of chlorine ultimately add salt, as Cl+ (oxidizing form of chlorine) becomes Cl- (salt). Bleach just includes salt, besides the oxidizing form of chlorine.

And yes, salt can corrode all forms of metal, if it's high enough -- including cover mechanical parts.

But, we wouldn't necessarily encourage people with 30K gallon pools to use bleach as a sole source of chlorine, long term. We just usually recommend it, on first contact with a new pool owners, since it's unlikely to make a pre-existing condition much worse. By contrast, adding cal hypo to a pool with high Alk and high Calcium can turn a pool milky looking, and using dichlor on a pool with high stabilizer can make a high CYA level higher.