Charles shows much more flex and does more side to side running, so I don't like that comp either. Miller ran like a scatback in college, but became more of downhill runner in the NFL. The Charles comparison stems from both their speed and acceleration, especially after contact, not really running style.
I don't think being stiff means a RB sucks, it just means they have limitations and might need to be in a certain scheme.
Coleman isn't stiff either. You're going to have to define what stiff means because if you're describing how they actually move, you're not using the correct word. Just because RBs like to run in a straight line instead of doing a lot of juking doesn't make them "stiff". If you want to say "this RB does a lot of straight line running" that's fine. That's not "stiff".
You compared Cook to Henry but now say one guy is better in a certain area because his body is a different size. If you know that a different body size does things in a different way they why did you make the comparison in the first place? You're basically admitting it was a bad comparison without having to admit it.
I think Cook doesn't look as explosive this year as last, but it could just be not having as good of blocking resulting in not having as many long run opportunities.
It's great to see that he's being used more as a receiver and is actually doing a lot better in that department than a player who was thought of as an excellent receiving threat, Christian McCaffrey.