Any proprietary parts that handle the piston system have no upgrades. It's Stags special system. And replacement parts are the same, Stag only.
Which means it's tough to build a Stag piston gun - no parts from anyone else. That's fine for a lot of shooters, the AR is the exception to the rule that most guns are a Make Model specific thing. The Rem 700 and Ruger 10/22 have a lot of aftermarket support, once past those pickings are slim to none compared to the AR market.
The majority of piston guns on the market in the last five years still in production is very low. Those owners are now facing no repair parts in the long term. If there was a standard pattern they all used, it might help. There isn't.
And that's not even discussing whether piston brings any improvements for the complexity and price. I'll just say that some makers build things because they sell, which is all they are concerned about. Sales. How you determine the gas is trapped and propels the action is up to you. Most of us go with the 45 year old proven pattern.