It should be noted that there are many different kinds of startups. Bootstrapped startups vs. investor funded, for example. Those will lead to very different experiences.
I’ve been fortunate to work with a bunch of wonderful, experiences, well-organised startups who have been among my best clients.
Painting all startups as inexperienced and disorganised is a bit rough. Those bad client qualities can be found anywhere. They aren’t more common in startups than they are in other small to medium businesses.