Clearly, rollerskiing is more specific to training on snow than any other training technique. Ski walking and bounding come very close, but rollerskis can be used to practice doublepoling, kick-doubelpole, and the various skate techniques.
But not all rollerskis offer the same benefit. Some rollerskis do behave fairly similar to snow skis, but others can actually hurt your technique.
Pedalling on skate rollerskis
The worst technique problem associated with rollerskiing is picking up the habit of "pedaling". Pedaling is caused by heel drop or excessive distance between your heal and the rear of the ski. In order to clear to pavement as you return the ski under you, you must pick up your foot by lifting you knee. This constant push out, knee up, knee down, push out is reminiscent of bicylce pedaling.
Roller ski modification with rubber band to keep the boot close to the ski.