Yes I too have heard that said however, I can only go on my own experience, which is that I have been wearing a Philadelphia collar, with washable liners replaced daily (essential), for nigh on 9 years, 24/7 and in my case, each day when I remove the collar for cleaning, I have the same level of use of my neck muscles, as I had 9 years ago. In fact my neck feels stronger, much less painful and far less fragile, so one could say they are better than 9 years ago.
I do appreciate that I have never spent more than a few minutes without the collar though, so perhaps am not qualified to comment on the affect that longer stressors would have on the head/neck but with the way I feel, I can’t see it being a problem, if ever I’m lucky enough for the ligaments to heal effectively. I would never regret wearing this collar at all, for it’s brought me great comfort, where for years there was only pain. Granted, it was cumbersome to start with but that’s of little consequence when you consider the great benefits it brought.
Your head and neck can still move within the boundaries of the collar, you don’t just ‘slump’ within the collar, it’s just that your movement is restricted and the collar guards you against many possibly dangerous manoeuvres, so it’s functionally supportive.