I think the chance of a Puma rolling over during taxi is very limited - however the chance of one rolling over on a 'heavy' landing caused by any sort of power failure, mishandling is almost guaranteed.
I know someone who has had hard landings due to power 'failure' in both a Wessex and a Puma. In the Wessex incident he had a little sideways movement on landing and was full of troops - the ac stayed upright. On the Puma he landed with only forward movement, reasonably light etc but went over on its side - he states to this day that had the second incident happened in a Wessex it would have stayed upright!