I think that the weak claim: "Hard work contributes to success" is supportable but the strong claim "Hard work guarantees success" is not supportable.
Maybe I'm overly logical and can't shake the whole "Necessary and sufficient" aspects of causality [1]. The charitable argument is that working hard can contribute to success but it is neither necessary nor sufficient.
It’s a half truth at best, which many regard as a kind of lie. Hard work, even smart hard work, is often necessary for success. It’s often not sufficient by itself.
With hard work you can be successful in the sense of being able to afford a basic living or being moderately middle class - which is enough for many people. The deception which looks like a lie to some is in implicating that one percenters got their wealth from hard work - most of them didn't.
AFAIK, statistics show that in industrialized Western countries the wealthy people who are seemingly successful (because of their wealth) have inherited most of their wealth or at least benefited extremely from improved conditions provided by already rich parents. In contrast to this, vertical social mobility remains fairly low and the overall gap between poor & middle class and the richest has been increasing in almost all capitalist Western countries since WW2.
So it's not literally a lie, it's just sometimes kind of dishonest and misleading. In most professions it's practically impossible to get rich from hard work.
"With hard work you can be successful in the sense of being able to afford a basic living or being moderately middle class"
There is plenty of empirical evidence that this is not really true. One of the pernicious things about the typical American narrative of success is that it often gets used in the contrapositive. This lets people feel good about discounting anyone viewed as "not successful" (whatever definition you want) as having been personally responsible for their own difficulties, so they can be safely ignored.
If you take the fairness out, that phrase is worse than meaningless. It applies to slaves: those who work harder are rewarded to some extent. Does that mean a slave society fulfills the ideals of meritocracy?
If you keep it as it is, it is an absolute statement as in "I can guarantee that every single time any random person works hard, he will be successful after time X" which is clearly empirically false because ignores all the (extremely numerous) boundary conditions.
The problem is exactly that, there are people who think that it is an absolute statement so poor people, unhealthy people, people in bad life conditions, etc are so just because they didn't work hard so they do not deserve any help.
Maybe I'm overly logical and can't shake the whole "Necessary and sufficient" aspects of causality [1]. The charitable argument is that working hard can contribute to success but it is neither necessary nor sufficient.
1. https://en.wikipedia.org/wiki/Causality#Necessary_and_suffic...