The short answer
Emily Kramer, who led marketing at Asana and Carta and now runs MKT1, names the number one mistake as hiring someone with too many firsts. She scores candidates on six dimensions: hands on execution, owning strategy, early stage startup experience, your specific business model, people management, and expertise in your most critical function. A first marketer should already have four or five of those, leaving one or two genuinely new, because with too many firsts the learning curve is simply too steep to survive. Her preferred shape is a pi shaped marketer with two real depths, most often product plus growth. Her second model explains why so many marketing hires disappoint: marketing is fuel (the copy, content and creative) plus engine (the channels, tooling, targeting and measurement), startups almost always over index on one, and hiring for the half you already have does nothing.