I have used the link multiple times with no real troubles. The first time I used it, the cast member couldn't find me in the system but a second cast member found me in just a few seconds. The other times have been perfect. I have been invited a few times and following the link in the invitation was no different from following the link here. (I had no need for the invitation because I had already set up my reservation)
Cast members will tell you that you have to be invited because, technically, you should only be able to get the fastpass if you are invited. They can't just go around announcing this link to everyone.
One thing that can happen, and what I believe might be happening to the people who have been turned away, is that after you go through the process of making your fastpass selection, it is not saved in the system. There have been many people who, after they confirmed their time, found that the system didn't actually confirm anything. If you make your reservation successfully, you will be able to go back into the system and see it. If it doesn't save, going back into the system will only let you select a new time. Make sure your reservation is actually confirmed before going!!
Also, I think some of the people who said they were turned away weren't staying on property. A few talked about needing to use the fastpass+ kiosks and not being able to use the app to change fastpass selections. Both signs of an off-property stay. I don't have any proof of this (just what I gathered from various posts), but you have to be staying on property to get into the system.
To summarize - stay on property and make sure your reservation is not only confirmed, but also viewable by going back into the system to check that it is there.