The small niche area of Nigh Net Worth insurance is built on relationships. A good account handler from a smaller broker is likely to achieve a better result than a poor account handler from a larger broker.
Many standard policies apply the average clause to their policies, meaning that any claim settlement will be made less the same percentage of under insurance. Worst case scenario, your policy could be null and void. A good broker can guide you through the pitfalls of under insurance and can pair you with an Insurer that offers free appraisals.