I would say it is relevant if you used different methods for each sample, but I understand why the author would omit "various" - it's too ambiguous.

I thought replacing 'quite difficult' with 'difficult' was unfair because I have always supposed quite to mean very or extremely when used in this manner.

Later he cautions against adding 'very', so he's consistent at least.

what about slightly difficult. It's basically a language denuded of gradation.

I think absolutism on this point is a bad idea. But I do think that many people, including myself, overuse qualifications in contexts where they don't add anything. In the example of a close match, what distinguishes a "very close match" from a "close match"? Better to omit the "very" unless you can quantify it, or otherwise make it clear what it adds.