My own limited inquiry into the matter (collecting general opinions from people who have learned foreign languages and ordering them by subjective difficulty) seems to generate a list where difficulty correlates strongly with distance from Latin. I suspect this is related to the way languages are taught, rather than the way they are structured. Our formal grammars appear to fit Latin very well, probably for historical reasons. Here's some ideas:

1. Finnish is known worldwide as a difficult language to learn. It has an extremely complex grammar (when put in Latin terms). Finnish-born children acquire Finnish in about the same time Spanish-born children acquire Spanish, but Spanish adults have much more difficulty learning Finnish than Finnish adults have learning Spanish. This suggests that teaching methods might favour Latin-like languages. A further supporting data point is that foreign people living in Finland who take Finnish courses do better at tests than people who don't, but take much longer (many years) to start actively using Finnish in casual situations (conversation) and are never as comfortable as people who acquire it without courses. This might of course be a factor of what kind of people take courses.

2. Finnish sentences are structurally some of the simplest, possibly the simplest of any European language.

3. The European languages furthest away from Latin appear to have the most complex case systems. Cases appear to be a hack to fit a language into a Latin grammar. Cases in point are Russian, Hungarian and Finnish.

4. Constructing a Finnish grammar that is almost entirely correct is possible without the use of cases.

5. This sort of grammar is more than sufficient for everyday casual communication.

This leads me to think that people who take Finnish courses might be too afraid of making mistakes to actually communicate. People who don't know they're "doing it wrong" focus more on the result of communication rather than the formal correctness. I know I am consciously ignoring a lot of the grammatical rules in German, but I can communicate in it freely. Native German speakers may get confused by me switching cases and articles at times, but I am always understood. I tend to prefer German to all other languages I know (all of which I have used for many more years than German) when speaking to native German speakers. I wonder how the list in the OP would look like if the target was a "conversation-sufficient" subset of the language. This would be much more meaningful than "general proficiency" (and much less testable).