Adrian Dingle, Author of Crash Course AP Chemistry prep book
The answer to your question in one sense is very easy to give, and in one sense basically impossible!
First the easy part. I absolutely defy anyone to tell me that AP Environmental Science (for example) is a harder exam than say, AP Physics C E&M - I just don't think it's a contest, and looking at the intellectual ability of the two groups that generally take those exams is a dead giveaway. It's also reflected in the way that some colleges view AP exams in terms of granting credit, in as much as they don't necessarily treat all subjects in the same way. So I would say the answer to your first question in a general sense is a resounding 'yes'.
On the other hand of course, your personal aptitude is actually what is going to determine how easy or difficult that you find any given AP course/exam; in that respect there are different answers to your questions for different people, and no one, simple answer that fits all.
One interesting resource is Total Registration that lists AP Score distributions by subject since 2011. It is sortable, so allows you to see which exams had more/less 5's etc. than others. HOWEVER, I would suggest GREAT caution when reading the data there, and be careful about drawing too many definitive conclusions. A couple of examples of what I mean;
In 2015, the subjects with the highest proportion of 5's are Chinese and Japanese. This is almost certainly because there are a LOT of native speakers that take these exams, and not because they are 'easy' exams!
Calculus and Physics C also appear high on that list, again, not because they are 'easy' per se, but almost certainly because it tends to be mainly intellectually stronger students that take those exams.