Any paper can prove to be difficult if u aren't well prepared for it. I personally found P4 to be the most difficult paper of ACCA...its not that the syllabus is very difficult...its just that the examiner asks outrageous questions in the paper.
P2 can also prove to be very difficult as well...because its syllabus is huge.
In the fundamentals level, paper F5 n F8 are generally considered to be the most difficult papers.
But, no matter how difficult the paper is, u can pass it in ur first attempt, if u r well prepared.
i do not consider F4(UK) to be very difficult. The syllabus is no doubt very large and the language is at times difficult to understand... but, if u analyze the past papers, most of the questions in the paper are repeated after a session or two...so most of the time, u know what the examiner is gonna ask...the F4 examiner ain't creative at all.
I can assure you that i NEVER read the 2.2 (now F4) study text and never solved a question from the revision kit. I only read and understood my teachers notes and then memorized all the official answers from the Past papers...and then i solved all the past papers under time limitations... I scored 79% in 2.2(UK) (now its F4). Thats why i normally advise ppl to keep an exam focused approach in their papers.
A part of the paper will test the application of your knowledge...but a major portion of the paper can be easily managed through rote learning.