From an optimization perspective, imperative behavior can be introduced if the functional API is not affected. Caching systems fall into this category. The functional benefits and freedom of usage is not impeded; the difficulty of creating and optimizing a caching system is the same as any other project.
Any quality from a project comes from discipline. Even purely functional programs can become unwieldy spaghetti with copy/paste code, memory leaks, etc. It's less about the programming languages, and more about proper experienced perspective of what to do and how to design.
I think that's where the article really comes across as tone deaf. If you're going to write an article with a title like, "Mostly functional programming does not work," then you better be prepared to demonstrate that mostly functional programming fails accomplish the thing it sets out to do. Instead, what he really demonstrates is that he fails to understand that there are desirable things about functional programming aside from purity and laziness. Which is in itself a rather fascinating thing to miss considering that the majority of functional languages are neither lazy nor pure.
Normally I've seen mostly functional programming being sold as a pragmatic solution that merely gives developers some tools that are helpful in certain situations. It's deeply uncharitable to suggest that users of multiparadigm languages are so naive as to believe that the mere presence of paradigm A erases the pitfalls of paradigm B.
I'm also deeply suspicious of any language evangelism that makes a whole lot of noise about the pitfalls of other languages and then implies that one's own favored language has no pitfalls whatsoever. This one just comes down to Occam's razor. Which is more likely: That your favorite language just happens to be the first perfect, wart-free language ever invented, and its relative lack of popularity is just because the rest of the world can't recognize its blinding awesomeness? Or that you've become so attached to your favorite language that your perception of it has become deeply biased?