All about individuals, both the child and the parent, actually.
Some difficult toddlers grow up into easy teens, some stay difficult, some easy toddlers become difficult teens.
Parents differ in what they find hard: some parents find it incredibly difficult to deal with teens talking back and disagreeing with their views, others quite enjoy it as long as it is not too rude. Some find it hard to know how much independence to give, others find life is easier when you are not quite so closely bound up with each other.
I was by all accounts the toddler from hell, but I was quite an empathetic easy-going teen, and in fact spent a lot of time supporting my mum who was having a difficult menopause.
Dd was quite a difficult toddler, suffered severe anxiety and health problems in her teens so that was difficult but she herself was a very pleasant person (still is).
Ds was an easy-going toddler, grumpy pre-teen and is now an easy teen.
I am probably the kind of person who gets a lot out of the teen stage: I enjoy discussions and don't worry very much about losing authority. Otoh though our toddlers were great fun, I did find it a strain to be clung to all the time. Tantrums I could cope with, not the endless clinginess and whining.