I've heard this myth about how certain languages are more difficult than others. For example, many people seem to believe German is more difficult than French, but then French is more difficult than Italian, and Italian is more difficult than Spanish and so-on-forever.
How do you know you're debunking a myth, here? Specifically, what evidence is there that some languages are not
more difficult to learn than any others? Have there been any studies done? It's a devil of a hypothesis to actually test!
Of course, it may be that most natural languages have turned out to have roughly similar levels of difficulty overall, and I can even think of a good reason for this: because roughly similar creatures (humans) speak them. For instance, it's commonly noticed that English has a massive vocabulary but makes up for this fewer features like grammatical gender to be learned and applied.
But I strongly suspect natural languages do indeed differ in their absolute level of difficulty. At the very least, two conlangs could theoretically be designed from one language as a base, with one conlang having its vocabulary, phoneme set, number of cases and tenses, exceptions, etc
. reduced, and the other increased. Would the former not be easier to learn, and the latter, harder to learn, irrespective of one's L1? That was the whole notion behind Basic English