I think gentle parenting, in my experience, can result in children growing up feeling very insecure and can affect their confidence later in life.
Children need to know someone is in charge and they are safe. Otherwise you are essentially putting so much responsibility and decision making onto a young child and expecting them to cope with it. They can't. And that is why they struggle to handle their emotions when faced with something they are suddenly not allowed to do (school, work, social situations).
As a parent, that responsibility should be on your shoulders, not theirs.
The dc's I have known growing up with my own who seem well adjusted and confident and basically nice to be around, are those whose parents had a varying degree of strictness, but there were rules and boundaries in place. I guess they were able to simply be kids without having every decision thrust upon them about how to behave and what to do and what do they want to do.
The ones who I would consider were gentle parented seem very insecure and hide that through over bravado, selfishness and attitude of doing what they want. But you can tell they have quite low self esteem underneath it all. I think deep down they just wanted someone to care enough to make them feel safe and not put every single responsibility or decision onto their little shoulders. They also openly have zero respect for their parents, especially their mothers. One calls his mother a bitch on regular occasions and she has no control over him at all (he is 12). Its quite sad to be honest, as they just are not liked very much by anyone and at the end of the day it isn't their fault.
Thats not to say it doesn't work for some people, this is just my experience. I guess different personalities of children would cope with it differently and turn out differently.
I do think when children are never told no or to be taught that sometimes they need to give and take or put others first, it will produce children who will ultimately struggle in life when things cannot go their way. Which is not doing them any favours at all, quite the opposite.