To get straight to the point, that thing is: When you shouldn’t use them.
Their goal is to make money, and the money in your pocket is what they want. Not your success, not effectiveness. Money! The more the better!
Not everyone is yet at the stage where it makes sense to start development of their product, 2 or 3 questions are enough to figure that out.
But for a development agency that’s usually against their own best interests. They want development to start because that’s when their billable hours also start.
There are a few that will take their customer’s success before their profits, but these are few and far in between (ask for recommendations if you want!)
I’ve heard the “I paid $150,000 to an agency and got nothing” thing so many times.
But I also heard the “I paid $150,000 to an agency and they built exactly what I asked for yet nothing works how I wanted it to” thing.
It’s very very hard to understand how building a product and software engineering works if you lack the relevant background, and there are many agencies waiting to profit from your ignorance.
The best way to protect yourself against predatory agencies is to find yourself a CTO or fractional CTO.