Thermodynamics is not any more useful than quantum mechanics in understanding obesity. It is moralizing disguised as an invocation of natural law.

Mm... I guess what this would be a case of I agree with the connotations of what you're saying, but not with the explicitly stated form, which I'd say goes a bit too far. It's probably more fair to say "energy-in - energy-spent - energy-out-without-being-spent = net delta energy" is part of the story, simply not the whole story.

It doesn't illustrate the ways in which, say, one might become unwell/faint without sufficient energy-in of certain forms, even if one already has a reserve of energy that is theoretically available to their metabolism, for example.

It's probably a useful thing to keep in mind when trying to diet, for those that can usefully diet that way, but it's not the whole story, and other info (much of it perhaps not yet discovered) is also needed. (And certainly using it as an excuse to moralize/shame is completely invalid.)

But I wouldn't call it useless, merely insufficient. What is useless is to pretend that there aren't really important variables that can influence the extent to which one can usefully directly apply the thermodynamics. (People who ignore the ways that other variables can influence the ability to usefully apply the thermodynamic facts and thus condescendingly say "feh, just eat less and exercise more, this is sufficient advice for all people in all circumstances" are, of course, being poopyheads.)