This is utter rubbish. Nobody learns to speak any language as an adult without an accent. That has nothing to do with phonological complexity, that's just how we're wired. Spanish may be easy to describe on a blackboard, but don't tell me that people who learn Spanish as a second language later on don't have accents. And languages don't succeed because of linguistics, they succeed because of politics. As someone whos…
Some things are really hard in Italian like the GL in aglio (garlic), but for example most Romans tend to turn it into a semiconsonant (like the first i in ieri). So basically you'll be understood.
Contrast that with French, that is also mostly phonetic but has a lot more vowels (around 17), that is also a language that is usually much harder to learn to speak as an adult (but it's not so bad if you just want to read. And maybe that is why everybody has this stereotype of the rude Parisian that never tries to understand a foreigner speaking French (though there might be more to the rudeness of Parisians).