When players run, they puts downward pressure on the surface in order to push off it. If the surface tends to "cushion" more than say a firmer pitch (like dry soil/grass) then that would mean effort in the pushing muscles (e.g. quads, calves) would be less efficient......so yes, I'd agree.....but then what if players were to play on a softer surface such as wet soil/grass? There'd be the same inefficiency when running as there is on the 4G pitch?
It's not as simple as that, although the concept is not far off.
Surfaces behave differently, things that strike those surfaces behave differently!
For example, if a tennis player exclusively played and practiced on a hard court then entered a tournament on grass they'd be at a massive disadvantage both in terms of the ball reaction to the ground and their own foot strike on the surface.
Widnes players will have a very well developed musculature, tendon and even bone physiology which copes perfectly well with a grass pitch. They've got this from years of playing and practicing on grass and playing every other week on a grass pitch won't be a disadvantage. They're also benefitting from the physical familiarity of training on their pitch which in turn will strengthen their physique and improve energy systems to cope with that surface.
Visiting players, even if they've trained on a 3G for a while will not match the familiarity, either physical or mental, that Widnes have with that surface
The coefficient of restitution affects the players footfall and cadence as well as the ball, it's different than a grass pitch and that's what will tax and tire different muscles of the legs and back in particular. A player who's more used to the surface will have better conditioned their body to cope with the reaction from the ground. That in turn will fatigue them less!
Then there's the effect of the coefficient of restitution on the ball.