Contacts are your friend, despite what some folks may claim ("Don't take contacts, since Missions hands your Contacts left and right"). Legwork can be a big, and important part of Missions, and the core NPCs in Missions tend to be from a fairly narrow field of business. Service Workers (Cabbies, Bartenders, Bellhops, Desk Clerks, Secretaries) are often better sources of information than bigwig fancy fixers and corporate mages.
Ultra Specializing in any field is a problem. Combat is the least problematic, but even then, what happens when you're on a team of faces, sneaks, and non-combative characters who want to try and sleaze their way through the adventure? If all you can do is one single thing, you can easily find yourself kinda bored during an adventure if that thing doesn't come up for whatever reason, or if there's multiple people at the table filling that same role.
Bull