There have been a few games that were dissapointing when i backed them before, but more often then not i got exactly what i wanted out of it and often even more.
For every bad game like Castle Story or Spacebase DF-9, there are a lot very good games, like FTL, Rimworld, Darkest Dungeon, Stonehearth, Endless Space, Endless Legend, Dungeon of the Endless, Door Kickers and many more. All of these have been definetly worth the money i put into them, most of them even more, and some like Darkest Dungeon, Rimworld and Stonehearth aren't even out of Early Access yet. Stonehearth isn't even in Early Access yet, it's mostly for kickstarter backers right now, but allready enough fun to warant my purchase.
Quite frankly a lot of people seem to have been quite reckless with their purchasing decisions on kickstarter and early access and blame the entire business modell when they get burned. I am sorry to say this, but if you back a Molyneux game and seriously expect to get everything he promises you kindof deserve to get burned there.
The most important thing is looking at the developers. If they successfully put out good, community-funded games before you should be good. If they haven't then look at how the developers act and interact with their community. You should be completely sure that you can trust these developers before you back them. You can of course still get burned if developers really enthusastic, but just plain bad at developing content, like with Castle Story. But it's a lot less likely.
If the game's in Early Access it's even easier. If the game, in the state that it is in right NOW, is worth the buying price their asking for it then buy it. If it's not then don't. It's not going to vanish from Early Access, so there's no reason for you to buy it if it's not worth your money yet.