This is an interesting topic, with lots of good points made for and against Flexible Funding.
I agree that a project which depends on full funding needs to be careful about FF, for the reasons already mentioned: perk fulfillment, etc.
I also agree that there are projects which can really benefit from FF.
Here are some details about my campaign
http://igg.me/at/i-dreamt-of-trees which show how I'm able to take advantage of FF without having any risk of letting down my contributors:
—Before setting up my perks, I carefully considered the costs involved in fulfilling each one, including printing on demand, shipping, highest possible cost of creating a perk depending on numbers ordered, etc. I also considered Indiegogo and Paypal fees, as well as income tax I will have to pay on funds raised. Taking all of this into account, I set the contribution levels at a point where I can fulfill each perk while still leaving money on the table for my campaign, regardless of total contributions received. For example, even if I only had one contribution throughout the entire campaign (fortunately I've already had more than that...), I would be able to fulfill the perk and have some money leftover for the project.
—Regarding completion of the project: The book itself is already written (it may still get a little polishing, but other than that it's ready to go.) Funds raised through Indiegogo will be a huge help when it comes to publishing, but if I need to look at other funding options when all is said and done, I will.
I also agree that there are other benefits to FF even if you don't reach your goal. Having an Indiegogo campaign is a great way to build excitement for your project, and get the marketing ball rolling ahead of your project launch.
One final thought on FF: If a project fails to meet its goal, it's important to consider that the financial impact on the project will probably be less than the amount by which the goal is missed, as goals are generally (or should be) determined based on money needed to complete the project
plus the cost of fulfilling perks. Therefore, let's say a campaign falls $500 short of their goal. If perk fulfillment, fees, etc. would have accounted for 50% of that $500, had they received it, then in truth the campaign will only be $250 short when it comes to the actual project completion part.