I hear lots of small issues with the brass tube models, worst of which they don't perform as well as open bolt, and are most susceptible to cold weather.
As far as durability goes, the brass tube is a new design, few moving parts, things friction fit, etc.
The open bolt design is TWO
parts. A block that holds the air nozzle. And an air nozzle from a GBB pistol. It's a design that's proven in hundreds of pistols, it's old tech, and therefore reliable.
Plus it looks way cooler than the brass tube design, and isnt that what GBBRs are all about?