when you use '有冇...' in the question, the answer could be "No, ...don't have..." or "Yes, ...have..." it means it always use in a yes/no question but when you use '有咩...', it is a open question.

有冇is question You said yes or no 有咩is long question 香港有咩地方好食 How about delicious food in Hong Kong but you must said the location

有冇：Do you have something / have you done certain things

Eg.

你有冇溫書？(Did you prepare for the exam?)

有咩：There are several possible ways to use this term.

Eg. 1.

Mary "你跌咗嘢" (Hey, you dropped something.)

Peter "有咩？" (Did I?)

Eg. 2.

我哋仲有咩未做？ (What else have we not done yet?)

Eg. 3.

有咩問題？ (What's the problem?)

Hope it helps. Cheers!