Yes, it is true. Actually, it is even worse than that - some of them never actually go into production. One of the leading IT firms in Silicon Valley spent over 5 years and well over $100 million trying to implement a product from one of the top vendors and never did get it to work. Their CTO is now looking for another job. The only thing you can do is to write a really detailed spec and request a fixed price quote. There are some vendors such as
enterprisewizard and RightNowTech that will do a fixed-price implementation, but even they will need a detailed spec first. I attach a spec template that we used to get a fixed-price implementation, but you should check with your possible vendors that this format meets their needs. Actually, even if you are not trying to get a fixed price bid, you should put a spec together. It is a lot of work and a huge pain, but you will need it eventually and the longer you delay, the more the implementation will end up costing and the further it will diverge from what you really wanted.
Note: We need to convert Bridget's standard spec format into something that does not obviously originate from EW.