Credit card cash advances incur rather large fees and charge interest immediately. If you buy an iPhone with your credit card and pay off the balance at the end of the month, you aren't going to pay any fees or interest. Most of the time, you'll get some cash back, rewards points, frequent flier miles, etc. from the transaction. If you go to an ATM and withdraw enough cash to buy an iPhone, you're going to pay a fee up front and you'll pay interest immediately whether or not you pay off the balance at the end of the month. That's the primary reason not to take a cash advance on a credit card.
Because it is a very expensive way to get money, the fact that you're using a credit card cash advance will make you look riskier to your credit card company. People that are regularly paying a lot of fees to get cash are generally people that are very bad at managing their money and have no alternatives. Since it's much cheaper to buy an iPhone on a credit card than to get a cash advance and buy with cash, the credit card company expects that you're using the money for things that you can't put on your credit card. Things like gambling debts or rent payments for example. And if you're getting a lot of very expensive cash advances in order to pay off gambling debts or to hand cash to your landlord, that probably means that you're in pretty dire financial straights and that the risk of bankruptcy in the near future is high. You can expect the credit card company to charge you based on that assumption.
Whether and how that sort of thing goes on your credit report and influences future credit decisions is something that is country-specific. Access to more credit, though, should be a secondary concern. The fact that you're getting cash in just about the most expensive way possible and the reason that you're getting very expensive cash advances should be the primary concern.
