Estimated audience sizes? Nonsense.
Suggested ranges for bids? Sometimes change by almost an order of magnitude between submitting the ad details and checking the dashboard a few minutes later.
Preview your ad as viewers will actually see it to make sure you've got the image size right? ::hilarious laughter::
Figures for click-through rates? I don't know why you'd only want to pay for actual clicks to your site and not other things like people clicking like on your ad or your page. After all, someone liking an ad that will be gone tomorrow is surely worth as much to you as someone actually visiting your site.
What times will your ad run? We'll assume you're in the US even though you explicitly told us you're in the UK. And then we'll also price everything in dollars, so you pay a conversion fee on every bill. And no, you can't change this once your account is set up.
Just set it up the same way as last time, we were satisfied with that? Sorry, the system has changed again and you have to start over.
Maybe we could use a third party ad manager to try to overcome some of these difficulties? Sure, but they're all going to work a bit differently, and the options they present won't match what you see on Facebook's own ad management pages.
All of this is based on various experiences as a small business advertiser, and in most of the cases I'm thinking of the real numbers still worked out favourable despite all the problems and the frustration of trying to get everything working so the campaigns went ahead. But if Facebook treats larger customers spending serious money the same way, it's not hard at all to see why we hear stories like this from time to time and why legal action can result.