For instance: "The good-looking are on average 7% happier than the bad-looking."
...from which it draws the conclusion that being beautiful doesn't make much of a difference. But 7% doesn't seem like a small difference at all. Spending hundreds of hours for a 7% improvement doesn't seem like such a terrible deal to me. Of course it's a bad deal if that's time you're taking from other things that make you happy. But exercise, for instance, has the benefit of improving your health and creating happiness in its own right.
Likewise the fact that 30% of unemployed feel 10% less happy years after becoming employed again. A 10% reduction in happiness doesn't seem minor at all.
I guess if you have some absurd expectation, like some notion that a simple intervention will make you twice as happy, you'll be disappointed by the effects of all these various things. But a lot of these changes are pretty substantial, and they're not even taking into account things that I would expect to really affect happiness--professional accomplishment, one's love life, etc. People are complicated, happiness is complicated, and of course no single thing is going to instantly grant you contentment. That's no reason not to work for small improvements; the small improvements add up.
And I think this kind of fatalism is really pernicious, akin to writing arguments that suicide is a good idea or that heroin actually isn't very dangerous. Most people are sensible enough to dismiss them, but others develop this feeling of learned helplessness which really harms them. The authors arguing this are always hypocrites; they know that they'd be unhappy living on the street, that's why they keep going to work every day. But they're perfectly happy to confuse and harm others with their disingenuous rhetoric.