While that may be true, I think that that example was more on a complaint basis and not on a percentage basis. I'd just like to see something that let's people judge how likely it is that they will be credited, so that they don't need to fill up the forum with complaints. Some kind of offer rating system might work for this purpose too.
Whether you choose to do an offer or don't should be based on whether the offer is something which interests you, as per the ToS. If you are looking for an insurance quote, and you don't get credit, then you lost nothing in trying the offer. If you need translation software, yet it doesn't credit, you still will get use out of the software. If you don't enjoy surveys, you really shouldn't be doing them then in the first place, and then won't get upset if one fails to credit.
So many things can happen for an offer to fail to credit that a rating system by users wouldn't be a fair assessment of the offer's crediting rate. What if some users didn't read the ToS, tips, etc before diving in and have their browser set to not accept third party cookies? They go and rate the offer poorly because it didn't credit, when it really was their own fault for not reading up first. Or, if the instructions say to confirm an e-mail address and they never follow through. Leave another bad rating? Again, this would be user error/fault. I just don't see it as a fair assessment by having such a system. Ratings could be skewed very badly based solely on users just not understanding how to correctly complete the offer. Plus, the forum currently acts as a great resource for everyone to talk about offers in the 'Offers' section.
If users don't get credited for an offer, FC doesn't make any money either, so it's in the best interests of the site to make sure that people DO get credit for offers that they complete. Yes, we enjoy doing it, but we also do it for the money.
Okay, maybe a rating system wouldn't work out very well, but I think a simple thumbs up (offer credited) or thumbs down (offer didn't credit) would be fine. The majority of people read what the requirements are before completing an offer. Other reasons for thumbs down could be valid too (too many popup ads, intrusive, spyware, etc...) that could really improve the FC experience by cutting down on bad offers. I don't think it would be abused as much as you say it would. To cut down on abuse if the site admins deemed it necessary, people would be required to write a comment explaining why they gave the offer a thumbs down.
Or even better, there could be a "Report this Offer" button that allows users to file complaints against specific ads and just a simple credit counter. Then people could view both the number of people who the offer credited for and the complaints of those whom it did not. The whole forum system for reporting bad offers is inconvenient, it would be much more streamlined if it were done this way.
Just some ideas, let me know if you like them (or not).