As I understand it, you are talking about trust management? In general, you are right, any trust management of your funds will lead you to losses.
Why would someone who knows how to make money on trading take other people's money for buildup? Of course, only in order to reduce his risks in case of losses, but he will not lose his money, but yours.
If trust is the only problem then actually that can be solved as a lot of traders take your login but don't get access to your email and 2FA which are required in order to make a withdraw. So they basically trade in your account and have the money but they cannot withdraw it so that eliminates the risk factor completely.
But, speaking in general I would also feel tensed giving control of my funds to someone else just because he claims he has a better trading knowledge and sense and I should hand over my funds under him. I know there is no risk of scam but some foolish trades might lead to obnoxious loss.