Proper positioning and assembly being equal, both will function fine on a range rifle. On a combat weapon, it's imperative that any potential abuse that would rotate the gas block and create a terminal stoppage be prevented. The government specifies pins, because they require the barrel to be drilled to install them. That prevents any shifting or rotation.
Quite a few users of clamp on blocks have smacked them hard enough to have alignment issues with the port. As they are .061" to .080" on most barrels, it doesn't take much. It's hard to see misalignment that small.
Since a piston kit is supposed make it more reliable - which is a hotly debated point - I see no reason to use clamp on blocks and risk them creating a malfunction.
Piston or DI, clamp on blocks are range toys, not duty rated.
