I think French is notoriously difficult to understand from speaking language because they tie together so many words and sounds. Contrasted with a language such as Finnish, which is hard to learn, but relatively easy to understand and write because both pronunciation and spelling are just what you'd expect.
This may be a hot take, but I'd argue that there is no such thing as a language that's easier or harder to understand. Just languages that are more or less different from the one you grew up speaking. If they're more similar, then the learner can repurpose skills they already had from their native language. But that's not ease, per se, it's getting a head start.
To take French phonology as an example: objectively speaking, enchainement is an aid to comprehension, not an impediment. Now that I'm used to it, I find non-native speakers who don't do it to be harder to understand because they've effectively dropped an entire information channel from the language. Which isn't to say I didn't have a hard time getting used to it when I was learning. I absolutely did. But that's not because enchainement is inherently difficult; it's because I first had to un-learn some assumptions about the structure of language and how spoken word morphology works that I was bringing with me from my native language. And because I was being hindered by pedagogical methods that, in effect, try to hide the problem instead of solving it.
