Facile est grātiās agere prō beneficiīs, nec vērō quidquam difficilius esse vidētur quam beneficiōrum meminisse. It is easy to give thanks for favors, but in fact nothing seems more difficult than remembering favors.
Is this a decent rendering of the second clause in this sentence?
Fine, although you’ve (understandably) omitted the esse in your translation.
One of the things my students find hard to get used to is expressions like nec ullum, nec umquam etc., which of course are best rendered in English as “and no”, “and never” rather than “and not any” or “and not ever”. In Latin, it’s never et numquam or et nullus, as we would expect in English. And nec…quidquam is similar.
You do see these quite often actually. It sounds like you’re interpreting guidance from a prose composition manual as a hard and fast rule. The way some of them occasionally lay down the law makes that a forgivable mistake.