I'm new to this forum, but I have a few years of avian experience under my belt and some variety of knowledge from others that I've gained that I hope is helpful.
My Amazon was free-fed for 35 years until I got her. Her original owners said she was aggressive towards them all the time, especially around her cage. They also stated she rarely was motivated to come out of her cage, she was a "perch potato."
Everything that I have read and learned from working at a zoo and with other professionals has taught me that free-feeding is not the way to go for a number of reasons.
My primary reasons are lack of motivation and the harm to the health. My bird was very overweight when I first got her, and it took about 8 months of careful diet altering to get her down to what I think is a "healthy working weight." Where she was motivated enough to work for sunflower seeds and other nuts and reinforcement. After she was finally on a "normalized" diet, two feedings a day many things improved - and almost immediately. For example, she stopped becoming cage aggressive because she was no longer "guarding" her food bowl because there was no food in it longer. She also went from being a somewhat messy eater, to wasting almost NOTHING, and I mean literally nothing. On top of it all, she is highly motivated to do training - which I think is one of the greatest things you can do with your bird or any pet for that matter. It builds a great bond and it is just a fun time for both to be had.
Now, all this being said when it comes to some situations I believe in free-feeding. For example, multiple birds in aviaries. Birds will fight over food when they feel there is competition for food. I've seen it first hand and know of many stories of birds going at it. Plus, most birds in aviaries are highly active from flying through out the day and are burning the calories that they are ingesting. But, mostly because as a safety precaution is why I would have my birds free-fed in an aviary setting with other birds.
Hope this helps!