In England and Wales we have "complete testamentary freedom", but the Inheritance (Protection for Family and Dependants) Act 1975 gives certain classes of people; spouses, former spouses who have not remarried (and didn't have a "clean break" divorce clause), civil partners, children, anyone treated as a child of the family and people who cohabited with the deceased for the last two years of their life or anyone who was being maintained by the deceased have the right to make a claim against an estate.
The only ground for a claim is that the Will did not make "reasonable financial provision" for them. In reality an adult child who is earning, or capable of earning, a living wage is very unlikely to succeed unless the parent has been paying their rent, mortgage or school fees etc. A spouse would usually be awarded what they might have got in a divorce i.e. half of the estate.
What is important is to leave a letter with the Will explaining any inequality of division and the reasons for it. As long as those reasons are valid and not paranoid or delusional they are very unlikely to be overturned by the court.