Specialization is a real edge: an agency on its fortieth HVAC account knows your CPCs, seasons, and customer objections on day one. You're not funding their education. For competitive local verticals, that head start is worth months.
The mirror-image risk: your specialist probably serves your competitors. Ask directly. 'who else do you work with in my market?' and 'how do you handle two clients bidding on the same searches?' Exclusivity by metro is a reasonable demand; vague answers aren't.
A capable generalist with a rigorous process beats a complacent specialist recycling 2022's playbook. Whichever flavor, judge them on the same scoreboard. The specialist just gets less patience for a slow start.