I think I can help with a few of these subjects - I took Chem + Bio (both OCR) and Maths (Edexcel) and have friends who did Psychology (AQA) and Geography (Edexcel).
Personally the "OMG Chemsitry A Level is insanely difficult" moment happened just before my 2nd AS exam so I was prepared for a jump to A2. I found the organic side of things fine as it was mostly the content covered at AS but relating to different functional groups. NMR took me a while to understand but ask for help if you're stuck. The physical chemistry was almost all new content, some of which was OK and some was much harder and took a few extra hours to understand.
I found Biology my easiest subject this year in terms of understanding content, however there is a lot of it to learn and if you don't like ecology/biodiversity/plants/etc it sucks. The long A2 paper is frustrating in that because there is so much content to learn, different things are always tested every year and you can't learn the mark schemes (although the new A Level may differ).
Maths has always been my most difficult subject and A2 Maths was very very hard for me. It took me a long time to understand the topics and I had to put in a lot of extra hours. However, if you find Maths easier than I do and see the patterns in the numbers/problems it becomes much easier and some people sailed through.
Psychology was very popular at my 6th form and most people seemed to enjoy it and get along fine. There seems to be a lot of content to learn and my friends did tonnes of practice essays all year but for the most part they enjoyed all the topics.
Geography again seems to have a lot of stuff to learn and I'm definitely glad that I didn't have to do 70 mark questions! The biggest issue for these seemed to be timing in the exam - the prerelease information appeared to be well used so you would vaguely know what to write. Again lots of practice essays were written all year here.
I hope this was vaguely helpful!