Real plausibility issue for this as a class-action suit. They're going to have to show causation -in other words, that the injury wouldn't have occurred or wouldn't have been as severe with a MIPS helmet. That can't just be determined on a probability modeling basis, but needs to be shown for each individual case. It's going to be hard to find cases, and they're all going to present very different facts.
Anyone with a really good case isn't going to want to join the class and give up control of their own case to join up with probably weaker cases.