They really aren't. In terms of delivering current to an appliance, they are ok, but in terms of safety (other than the danger caused by the pins sticking up if you leave one on the ground, and then walk around with bare feet) the British ones are vastly better.

It's too easy to poke objects into US sockets, they don't grip the plug well enough to support the weight of power adapters, the cords themselves often only have a single layer of insulation. British plugs are designed so that if they are stuck halfway in, you can't get access to a live pin even if you poke a metal object between the plug and the socket, and it's highly advised against to mix phases in the same room, whereas (at least in Canada), it's not unusual to have a dual socket wired so that the neutrals are effectively the centre tap, and the live (hot) pins are on separate phases 180 degrees apart. So you think you're dealing with a 120V outlet but it's actually got a supply of 240V to it. That's off the top of my head.

I've done a fair amount of electrical work both in the UK and North America (Canada, which has has similar electrical hardware and regs to the US).