A UK bank got heavily slapped on the wrist for this sort of comedy
You transfer your £5000 debt at zero% - fair nuff you say i dont pay any onterest on 5K, however what they did not make clear was that any additional borrowing on the card was charged at a silly high interest rate. So whilst you may think zipydadodahday im off to spend another 500 sovs, the bank are jacking themselves silly, because any repayments you make comes off the 5000 at zero percent. The additional 500 you have just spent will continue to accrue interest at daftly high rates that would make a loan shark feel bad about it and offer you a loan of a tenner, until you have cleared your 5000 and then start to chip away at all the interest you have accumulated.
If you want to sward cards - look for a card issuer who pays off the most expensive loans 1st, a couple in the UK do now.
And yes they do know which dollars in which loan are at which interest rate, that 5500 is not one big fat happy loan at zero percent,