Decision rules
When a balance transfer is doing a useful job
The fee is smaller than the interest saved
Do not focus on the headline 0%. The transfer fee is part of the cost and is usually added to the balance.
The old card is not reused
If the old card fills again, the transfer has converted one debt into two debts.
The balance is clear before expiry
If the monthly payment needed is impossible, treat the offer as breathing room, not a solution.