I have a chronic illness. It would have been a real consideration in deciding whether or not to have a baby as I get a lot worse without sleep and pregnancy itself can also make it a lot worse. So I would have been quite concerned about how I would handle it and whether I would be able to care for the baby. However my first pregnancy was unplanned and I turned out to be one of the lucky ones where pregnancy actually greatly improves rather than worsens the condition. So I ended up coping very well and by the time my symptoms returned with a vengeance I had a good plan of action in place with my doctor and we were able to handle it better given the insight provided by my reaction to the pregnancy.
Then once we decided to have another I was quite looking forward to the break I was expecting from my chronic illness during the pregnancy. Unfortunately the second pregnancy didn't quite work out the same as the first . Though it still didn't make things much worse than they had been.

I think it really depends on the particular illness, how it is expected to interact with pregnancy, and what your own expectations are and what you're willing to accept.

Of course there's always a large element of the unknown, particularly with some chronic illnesses. I have postural orthostatic tachycardia syndrome and it's not yet very well understood, probably actually a few different things grouped together. While I was lucky and my condition greatly improved with my first pregnancy and was only moderately worsened by my second, I know someone else whose condition was greatly worsened by her first pregnancy to the point were she spent a lot of time in hospital and was not able to care for her son for his first year.