Recently my PayPal account was frozen without warning, quite worrying at the time and sorting it out was a bit of a hassle. It turned out it had been frozen because there had been an attempt to hack it.
Very commendable and reassuring but its processes for (not) informing me and the days it took to sort out cost me a bad mark on ebay because I didn't complete a sale. The seller wanted me to pay by PayPal or credit card through PayPal and I could do neither. In the end I contacted the seller, a trader, direct and, as the item was trivial and the sum involved less than £10, we agreed to cancel the sale, however ebay just registered this as a non-completed sale, even though it had been agreed mutually with the seller.
I think the biggest problem with PayPal is that their systems are robust but their processe for informing customers there is a problem and then dealing with it are opaque