i would first of all avoid the vpn providers who provide their service for free for the users. because here i am quite sure that the logs are stored and also used for other purposes.
with the vpn providers that offer their service/apps only against payment, you can also not be completely sure - but in any case safer than with the free variant. i personally use 'expressvpn' here.
Basically, all services that offer free VPNs also have paid packages. Free generally has certain limits and the primary purpose is for the user to try the service before purchasing. This can be considered a form of promotion and it does not sound logical that in that case they have "hidden" features. I guess they should show themselves in the best light and attract new users.
I'm not refuting your statement, I think you're right, but what I stated is an ideal scenario.