The PPL subjects just give a light overview to the core aspects of flying. To be honest, I did them just before my ATPLs and I don't think they really helped much. The quantity of information you have to absorb is huge and you need to be disiplined. I did a degree some years back and found the ATPL ground school much more demanding. Getting high scores in the PPL subjects will have little or no impact on the study time for ATPLs and completing all in 6 months is doing well. You'll also do exams every couple of months not at the end, so you wont have to remember what you learnt 6 months ago.
People struggle with different subjects, but here's my opinion:
Instruments - hard
Gen Nav - hard
Radio Nav - boring, but not too hard
both comms - straight forward
Met - hard
Aircraft General - hard
HPL - Interesting and fairly straight forward
performance - medium difficulty
POF - hard
Mass & balance - not too hard
Law - boring, but not too hard
Ops - boring, but not too hard
Flight Planning - boring, but not too hard
Good luck !
Biscuit
PS Note the frequent use of the words "boring" and "hard" !