You hit the nail on the head with complimentary piece - and extending that to personality as well.
You need a mix of technical skills (engineering, product, marketing, etc) but also personalities (which manifest in different behaviors which are all useful & compliment each other - eg bias for action and hitting targets vs careful analysis).
I'm very strong in logical analysis and strategic thinking. My problem though is that I have a natural tendency to not talk to people. Ask me a question and I'm happy to answer, but I don't like to initiate conversations. Stefan is the opposite. Everybody loves him. He can talk to everyone and he's very creative in finding leads and building connections with other companies. But he's a really bad developer. Together, we're a really strong team. I'd say without him, I would still not have a strong network.
You need a mix of technical skills (engineering, product, marketing, etc) but also personalities (which manifest in different behaviors which are all useful & compliment each other - eg bias for action and hitting targets vs careful analysis).