WD-40 is a lubricant. It contains 25% of the same (or similar) oil that is found in all oil based bicycle lubricants. That oil is in about the same ratio of oil to solvent as well. It’s not going to melt your chain if you use it as a chain lube but, because it is a spray, it is easy to get it everywhere. Bicyclists generally don’t use spray lubricants for that singular reason.

Simple Green may have had a corrosion problem when it was first introduced but it’s current formulation has a lower pH (less basic) that isn’t a problem. Simple Green also isn’t all that effective as a degreaser. I’ve been using Armor All Wheel and Tire Cleaner lately but have more recently discovered Dawn Powerwash spray which works even better. I even used it on clothing to completely remove chain grease from some clothes. It is wonderful stuff.

Oil, being liquid, flows. Put it on and no matter how much you wipe it, it will flow to the outside of the chain until there is no oil left. It will also make a black gunky mess anytime you use it because of metal fragments that get entrained in the oil along with the grit that causes the metal fragments.

You don’t really need to lubricate the pivot pins on the derailers all that often. Excess lubrication is bad because it collects grit which can act to wear the pins. Go very fugally lubricating the derailers. They don’t really need it very often even if you are riding off-road in dirt and dust.