How do you increase it on those machines which appear to have zero resistance? I've seen those leg ones before and they just swing your legs, they actually reduce the work
Only useful for very old people. No use to fat people or children.
Here's the crux of the problem - you don't know how the machines work, so you've dismissed their effectiveness.
Ive used them a few times, the mechanism is linked to the seat so you are lifting your own weight to provide the resistance. Quite a good design.
Obviously, they aren't fully adjustable like machines in a gym, but they are outside and free, so what do you expect? I stand by my assertion that they are very useful and a good idea for the casual exerciser.