It's a penetrant/solvent (cleaning grime, flushing old grease)
AND a lubricant
Which makes it relatively ineffective at both functions. The lubrication will not last or work all that well and a real solvent like kerosene or a degreaser will clean stuff much better, and not also make them dirty. Like say you're trying to actually clean something, wd40 is actively trying to also combine that black gunk with oil and have it stay there.
As for something 'mild/easy' you can get quite far with a bucket of dish soapy hot water and that's less likely to fuck up your hands or give you cancer.
IMO it's specifically useful for flushing grit out of derailer pivots and de-gumming shifter internals, mostly because of it's aerosol nature, and because re-oiling/greasing those correctly without full dissasembly is not possible.
It's ok for hitting siezed bolts but i started using 'cyclo breakaway penetrant' which seems to kind of foam and ingress a lot more effectively for that purpose.
