I'm referring to 16 years of experience teaching language and seeing where the pain points were in acquiring English from Mandarin speakers. The irregularity of English grammar was never a particularly difficult point. The Chinese just sat and memorized, something they're good at from just their own orthography, given that it's almost, but not quite, entirely devoid of system.

What were pain points were conceptual pain points. Most people couldn't grasp articles and when they should or should not be used. (Esperanto has an article whose use case is bizarre.) Most people had a hazy grasp on verb conjugation, freely using whichever conjugation first passed their lips without subject/verb agreement. Declining for number was a pain point. Even the mildest amount of gendered language caused problems ("he" and "she" tend to get used interchangeably and fluidly, often switching between them in the same sentence). Verb tenses. Verb aspects. Both of these caused tremendous difficulty.

And Esperanto has all of them and more.

Would Esperanto be easier than English to learn? Of course! It's far more regular than English. But the point here is that while easier than English, it's not much easier than English because as a language at a conceptual level it is not that different from English. And then on top of that the consonant clusters (thank you Polish!) would render it nigh-impossible to pronounce. We're talking about people for whom the word "lonely" is a tongue-twister because of the switch between 'l' and 'n'. For whom the "str" in "string" is a pain point. And I've spotted Esperanto words with five-consonant clusters, four of them hard.

There is not much difference in terms of difficulty between learning English for Mandarin speakers and learning Esperanto because the difficulties come from conceptual levels, not practical. There are alien ideas in Esperanto (shared with English), and that's where the hard part comes. So the choice of a Chinese speaker is to learn Esperanto and get (generously) a million people (of eight billion) to speak with, or get (equally generously) 1.5 billion people (of, remember, eight billion) to speak with.

When that stark calculus is presented, the choice is clear: spend the little bit of extra work it takes to learn English and ignore Esperanto.

I'd be very interested in seeing your mentioned studies, incidentally. Specifically seeing who performed them (and what their methodology was). My guess is that they weren't professional linguists, and nor were they particularly rigorous (using things like self-selected subjects, etc.).