什么 + [X] + 不 + [X] + 的,[sentence]
什么 + X 不 X + 的 picks up something the other person has raised – whether it's tiring, expensive, convenient – and waves it away as beside the point. A comment or instruction follows. It's spoken and blunt; tone decides whether it sounds warm or impatient.
X is usually the word the other person just used, an adjective or a verb.
A pair of opposites can take the place of X 不 X.
Because it waves away what the other person said, it suits friends and family. To a teacher or a boss it sounds dismissive.
Without the frame, X 不 X is an ordinary question asking for an answer – the opposite of brushing it aside.