I'm currently on AWS for my site and in the process of researching alternatives. I share your concern of something going wrong and being stuck with a huge bill. Someone pointed out that 1TB of outgoing traffic from Amazon EC2 would cost $90. I'm fortunate enough that that won't obliterate me, but I won't be happy if that happens. I'd rather my blog get hugged to death. Going viral isn't worth $90 to me.
But I don't think DO really solves this problem either. They say they have spending caps in some of their marketing materials, but the finer print says that overage billing is $0.01/GB. Now that's a whole lot better than Amazon's $0.09/GB, but it's not a cap.
DO can say they have "predictable pricing" because in the vast majority of the cases the "free allotment" that comes with your droplet is enough, so you never see a bandwidth charge, you pay the cost of your droplet and you're done. So yes, it's more predictable because Amazon would charge you $5.23 one month, $4.87 another month, and DO charges you $5 every month.
But I'm not worried about the 99% case, I'm worried about the extreme scenario where I somehow go viral or get DOSed. And both options leave me exposed.
That's not to say DO isn't a better deal for the hobbyist than AWS. The equivalent of DO's $5 droplet will run you much more on AWS, especially if you actually use the bandwidth they're allotting you. And the big 3 do a lot of nickel-and-diming, which is a nuisance compared to the simpler pricing model of the smaller providers.