My company has those surveys. As far as I know they are anonymous but the problem is any feedback falls on your direct supervisor to fix. The first year they did it a lot of us were honest about it and our boss got absolutely wrecked over it even though he had very little control over the organizational problems we were complaining about so the next one we always just based it off things he had a say in which he's a good boss so the surveys no longer aligned with the company as a whole.