More reliable? Not really. Not in any realistically possible conditions anyway. (You don't plan on taking them to the moon do you?)
Easier to clean? Maybe, then again a direct impingement AR is not really that hard to clean anyway.
Like others mentioned. For the money you would dump on a piston AR. You can get a very nice DI rifle that will be just as reliable as the pistons and probably an overall better rifle. Plus have parts commonality with pretty much every other AR out there.
What happens when Ruger or Sig decide they don't feel like producing their rifles anymore, and yours breaks. Can't just swap in a replacement from another manufacturer. Since the all have their own proprietary system.