for me its that its not at all long lasting. I guess it's fine as a cleaner, but even light mineral oil hangs around longer.

oh right, it also seems to leave a gummy residue, which is really not great for machine tools

yeah, most of my use-cases for classic wd-40 have always been getting things unstuck rather than long-term lubrication. The lubricating action tends to evaporate with the solvent(s) and leaves, as you've pointed out, the famous gummy residue that is good for keeping moisture out but not at being a lubricant