This is a great formalization of a three-resource model - time, money and mental energy - which clearly gives much better answers than a two-resource model of only time and money in cases where mental energy is a relevant resource, which is often true.

Despite that, it still feels woefully incomplete/simplified, given how important it is to get something like this right. One of these is that there are lots of resources (N is large) and trading is not cost-less between them; you have to draw the line somewhere, however, and can draw it as needed by a given exercise. I think more importantly than that, it comes down to the fact that while money is fungible and savable, time and mental energy (and many other key resources) aren't. Resources that are use-it-or-lose-it, but vital to pretty much everything, like time, have highly variable marginal value, which makes the calculations very different than described in the paper. I'm going to try and expand/formalize this concept more.