I have always seen the Internet advise against it. Although I have made profit in this way.
You were lucky, that the price has not fallen by the end of the repayment period...Just imagine, that you have no funds to buy bitcoin, just a credit card, with the credit line of e.g. $1,000
You have bought 0.33 bitcoin on 1st August, (price was ~$2,900), so you paid $957 with your credit card.
To make it easier, let's say you have 1 month period, when you can pay back your credit without interest.
On 1st September, bitcoin price was $4,850, it means you had $1,600 worth bitcoin. When you have paid back your credit, the net profit was $643. (nice profit for one month investment...)
But if you have bought 0.2 bitcoin on 1st September (paid $970 by credit card), and let's assume that on 1st October the price of the bitcoin will be about ~$3,500, that means that you have to pay back the $970 credit, but your bitcoin worth only $700 the net loss is $270. You have to pay this back from your own funds, or, if you have no funds available, bank starts to calculate interest from the 1st September, and on 2nd October, your $270 net debt starts to increase because of the ~30% interest rate (p.a.) of the credit card.
