I've been in the brewing industry for several years in one capacity or another.
Capital, business knowledge (employee hiring, wage laws, labor laws, taxes, marketing, advertising, AP/AR, accounting, sales, etc) brewing science to scale, consistency, customer service, market placement, product placement, taxes, regulations on federal level, state level, local level, politics of being in business. It all comes into play. There are certain people that are cut out for it and most people aren't.
I've seen and worked in more breweries that have come and closed than still exist today. The surest way to ruin a great hobby is to make it a business.
I've seen more brewpubs fail than breweries. In breweries you only focus on the beer. With a brewpub, you focus on a restaurant and a brewery. A restaurant is a headache of its own and you're combining that with a brewery now. Both are separate management problems that must be managed successfully in order for the pair of them to be successful and maintain or grow that success.