I've seen multiple situations in TV shows, fictional and nonfictional, where wills or other expressions of post-mortem wishes have been contested for reasons that seemed trivial.
One was a man who named his friends, a successful married couple, who loved and were loved by this child, and who he believed would be better guardians than his parents (the child's grandparents) to be his daughter's guardians, but once he died there was a legal battle over custody as this man's parent, the child's grandparents, thought they could provide a slightly "better" life due to being more wealthy.
Is this enough of an argument? Is a person's dying will not the final say unless extenuating circumstances are proven?