That was just one example but you win I guess. What I had in my mind was for a Spanish speaker it's probably easier to learn a hypothetical Romance language that doesn't have grammatical gender than say French.
So first, I would say that the notion of ease of learning a language is a purely relative one, which is what GGP probably meant. It is easy for an Estonian speaker to learn Finnish, a Dane to learn Swedish, etc., but quite difficult for an English speaker to learn Finnish. As another example, Mandarin on its face should be an easy language to learn because the grammar is very simple - there are no tenses, no cases, no gender, no inflection whatsoever. There's tone, sure, but this isn't the difficult part of understanding Mandarin for a native English speaker. Regarding grammar specifically, despite Mandarin seeming to possess a nominally simpler grammar, speakers use patterns that would appear alien to most speakers of European languages. For example, from section on cleft sentences[0]:
他昨天买的是菜
Literally, this means 'He yesterday buy of is vegetable', but the meaning is 'What he bought yesterday is vegetables'. The way the grammar in this sentence corresponds to its meaning may seem unusual to you, but it is effortless and natural to a Mandarin speaker. And these sorts of invisible grammatical features exist in every language. Most European languages descend from a common ancestor, so it is not so noticeable, and instead we notice the grammatical features that are different between languages: cases, gender, tenses, etc. And Esperanto, being based in these languages, also inherits these unnoticed grammatical biases. A Mandarin speaker learning Esperanto would have the same difficulty as an English speaker learning some Mandarin-derived conlang that 'simplified' certain visible grammatical features. Esperantists tout success in speakers of Asian languages learning Esperanto, but there are many more cases of speakers of Asian languages learning English.
Anyway, I don't think things like gender matter so much when it comes to the practical aspect of learning a language, or even mistakes in semantic grammatical features like case or tense. A native speaker isn't going to be unable to understand you because you flub some of the grammar such as the gender of some noun. Consider when you hear non-native English speakers make mistakes - are you unable to understand sentences like 'I go to bank' or 'Yesterday I eat restaurant'?
So these are the two flaws of Esperanto:
1. Esperanto's grammar is still very much Romance + Germanic-like, so ease of learning it is relative to familiarity with languages in those families.
2. To the extent that Esperanto's grammar is simpler, it doesn't particularly matter from a language acquisition perspective.
Esperanto is still one of the easiest languages for Chinese to learn and the grammar is not very European; it also has elements mostly found in Asian languages:
The Chinese are actually one of the biggest supporters of Esperanto too, offering degrees in it from major universities, regular broadcasts on official radio and some years ago teaching it in primary school.
