Image source: The Motley Fool Warren Buffett isn’t afraid to make big bets on individual stocks. Believe it or not, around 70% of his portfolio is invested in just five of them. One of those is American Express (NYSE: AXP). At present, Buffett has around $23bn invested in the credit card company. The size of his investment clearly suggests that he’s very bullish on the company. That’s got me wondering whether I should buy it for my Stocks and Shares ISA. Three reasons to buy In my view, there’s a lot to like about American Express from an investment perspective. For starters, the company has a great track record when it comes to generating shareholder wealth. Over the long run, the share price has risen significantly, generating healthy gains for investors like Buffett. Meanwhile, the company has also rewarded investors with consistent dividend payments. For 2022, it’s expected to pay out $2.08 per share in dividends, which equates to a yield of around 1.4% at the current share price. Looking ahead, there remains plenty of growth potential. Experts believe that over the next decade, trillions of transactions will shift from cash to credit cards and electronic payments. This shift should provide huge tailwinds for the credit card company, which currently operates in over 100 countries. It’s worth noting that Wall Street analysts expect revenues to rise around 11% in 2023. As for the valuation, it seems quite reasonable to me. For 2023, analysts expect American Express earnings per share to come in at $10.50. That puts the stock on a forward-looking price-to-earnings (P/E) ratio of around 14 right now. That’s lower than the US market average (and roughly in line with the FTSE 100 median). The big risk Of course, the big risk here is consumers’ financial strength. Unlike Mastercard and Visa (which simply operate the payments networks), American Express issues credit cards itself. This means it takes on credit risk. If economic conditions continue deteriorating, we may see more consumers default on their loans. This would have a negative impact on American Express revenues and profits (and most likely send the share price down). This risk shouldn’t be ignored. It’s worth pointing out that consumer credit reporting agency TransUnion recently said that it expects the rate at which US consumers miss payments on credit cards and personal loans to surge to levels not seen since 2010 in 2023. My move now Given this risk, I’m going to leave American Express shares on my watchlist for now. I can see myself buying this Buffett stock for my ISA one day. However, I’m a bit hesitant to buy right now due to the high level of economic uncertainty and the potential for loan defaults. All things considered, I think there are safer stocks to buy for my ISA today.