Basically, if you're gonna be riding aggressively then a coaster brake might not be for you. A good coaster hub will still stand up to the abuse, it just won't stop you so fast. If traffic in your area is not very bike friendly and is dense and fast moving maybe a coaster brake won't be for you. If on the other hand traffic is reasonable and courteous to bike riders and you don't foresee yourself having to slam on the brakes when you get cut off or etc then go for it. Once you're used to it you'll be able to stop safely and fairly quickly even with just a coaster brake. Not as quickly as if you had a front brake, but pretty quickly nonetheless.