[syllogism]

The problem is that the hiring risk is greatly amplified if they don't know the stack. If they don't know the stack, they say "Give me a month or two, then you'll see."

Well, in a month or two, what do you get? Who knows. Evaluating anyone from a resume is very difficult. They might be terrible, but you had to give them two months to find out.

If they claim to know the stack, a bad hire has nowhere to hide. They'll be found out much quicker, so you've lost much less.