I suppose this isn't suprising given that companies go bust all the time, it's just the quantities of money we could be talking about could put a serious dent in your bankroll. For that reason I'm going to stay away from any site that's slow paying or has reported issues, the risk outways the potential reward at the moment.
You may not have heard of Canbet, they were an Australian bookmaker who'd been around for some 20 odd years, hardly a new upstart company. They were offering rediculous bonuses last year, and then stopped paying, and have recently confirmed they're trying to sell anything of value (I assume that doesn't include the name which is as good as mud now).
There are lessons to learn, and you and I need to learn them. The best you can do is look out for the signs. If someone starts slow paying, beware (you can use the link to www.payout-times.com below to keep up to date with that, or for more obscure sits there's The Gambling Times payout reports page. Also, read this article on The Rise and Fall of Canbet. It's really interesting and will help you avoid being sucked into what had all the hallmarks of a Ponzi scheme by the time it was wound up.