At least so far, insurance companies are not very strict about telling the truth, unless it involves malignant problems, such as being required to have a very different physical examination, diagnosing hereditary diseases, or the basic conditions such as occupation and age are seriously inconsistent with reality, which will affect future claims.
As for cancer, it seems that insurance companies haven't really understood the family inheritance, but if they are afraid of taking responsibility, they should try to write it in.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.