It's not hard honestly, most of it is very simple. It's all about practice. But if you found GCSE Maths hard and struggled to get a 7, yeah chances are you'll find A-level Maths hard let alone Further Maths. If you coulf easily get 8/9 in GCSE and get 80-85%+ easily without practice, you'll be fine doing Further in most cases.
However, I and others in my class personally have found this one topic called Geometric Representations of Roots of Unity and Geometric series of Complex numbers to be extremely difficult, although the latter is alot easier after some practice.
Another thing you will find hard is Further Mechanics, but this is also extremely easy once you have enough practice and get some of the Physics.
Apart from that, everything else ia actually very easy especially differential equations which you really just need to memorise the method and it's basically free 12 marks or so. A lot of Further Maths topics for me have been long, not hard but long. As in the questions just take loads of steps or many steps but include tiny calculations multiple times which can give room for error to due silly mistakes of mis-writing a 1 or x, not mistakes due to your maths being wrong.
But yes in my honest opinion, the hardest topics are those 2 complex number topics. Everything else is very straightforward and if you're practice is up and going you will find it straightforward too.
Remember you don't need 100% to get an A*.
BUT IF YOU DO NOT WORK AT IT OR PRACTICE YOU WILL FIND THIS VERY HARD BECAUSE YOU WILL BE SOOOOOOO BEHIND. Further Maths is a topic imo where if you get behind then it's very hard to catch up coz the pace is strong, especially if you do 4 alevels and Alevel maths in 6 montha and Further in 12 months (like in my school).
If you don't practice, then yeah you will complain like the general population who do say Further Maths is hard. It's not, the difficulty is the same as A2 Alevel Maths, except you DO MORE MATHS.