Personal umbrella policies tend to be a very profitable line of business for insurance companies. In my 35 years of working for insurance companies, I only recall paying a few umbrella claims in my territory and few were for policy limits. The few I recall were just additional amounts over the top of the underlying coverage but not policy limits of the umbrella. I did see a claim where my employer declined to write an umbrella policy for customers that wanted higher limits than we cared to provide. That insured ended up with a major claim. Their grandchild drowned in their swimming pool and their adult children sued them for big $$. I don't believe the grandparents were even home, but cases like that are tough in front of a jury.
Insurance companies make money on umbrella policies by carefully underwriting them. They shy away from bad drivers, high-powered boats and other toys that are more likely to cause large lawsuits. Kids on ATV's going under barbed wire fences, falling off of golf carts, jet skis, etc. are always suspect as are trampolines and swimming pools.