I think of them like car mechanics. Some just do oil changes, air filters and wiper blades; some are good at routine maintenance and repairs; others specialize in certain makes and models; and a very few will take on vehicles that are hard to find parts for and costly to repair, but they enjoy working on those cars a lot and love to see them back on the road outperforming the rest.

eta: and some tell you things are broken that don't need fixing at all, or they are in over their heads and end up doing more damage; and by the time you stop giving them any more chances and listening to their "this should do it, trust me," you find yourself having to find one of those rare specialists who can fix a mess that you did not start out with.