Also, if you don't want them to be incompetent, maybe just have them in over their heads? Running a café/restaurant, all in one place, is one thing (even with gift shop).
Branching out into parties and weddings requires more people, transport, equipmen and furniture if they do off-site events, lack of total control over the menus, (and in a smaller place, how do you find trained help? And, fancy stuff for the special event menus has be ordered in, on short notice.)
Tighter cost management and bookkeeping is necessary - tough if they've been used to a set menu for the restaurant, and doing stuff on the back of an envelope.