It depends on the individual. Some may find learning difficult, and struggle with drill/weapon lessons etc. Other may not be as fit as some and find the exercise harder. Others may be a sleepmonster and find the Exercises hard on them.
Nothing in basic is difficult in itself. The difficulty is in changing from an idle civvy tosser into something resembling a soldier, and being able to do all the things that are wanted, all at the same time, and to perfection, without really having to think about it too much... and I do not mean 'don't think' I mean it being natural and instinctive. And try to spell properly. Good luck.
Some may find that they can't stand being with other people 24/7 They generally don't last very long as they tend not to be good team players. You really are going on a journey of discovery, enjoy the ride.
The hardest bit for me was having to stand to attention on an icy February night (1800-2100 then 0500-0700) because my section commander didn't think we were performing well enough. Despite the fact that we were the best at everything.
Mind you, he was from 2 Para and called Psycho Bob for a reason.