People come into positions of authority for very varied reasons. Some might be very technically apt, others might have earned the trust of the founders or some were simply the only people available in a period of rapid growth. You absolutely cannot trust human nature, abuse and incompetence is always a risk and startups are not magically immune.
What might happen in a small company is that a bad manager is so obviously inadequate and has such a significant relative impact over the company that it's easily spotted and fixed without a formal system. But there are no guarantees, see for example the case of Sunny Balwani, a painfully incompetent software guy running the engineering department of a biotech startup, simply because he was the boyfriend of the CEO. He clung on as Theranos valuation balloned to a $10 billion and only got out after the company failed for essentially tech reasons.