I haven't carried a deco bottle or three with my Transplate harness so I can't say without assurance that the plastic buckles won't fail. However, I've done some stints with the US Army paratroopers and have jumped out of airplanes with ruck sack that have same types of buckles with said ruck sack slamming into the ground upon landing, and with said ruck sack loaded up to at least 70-lbs worth of gears. None of those buckles ever broke. Said ruck sack also went with me to Alaska, Germany, Korea, a coupla joints in the Middle East while fully loaded down with gears and said buckles tested by being sat on, thrown off vehicles, rolled downhills (my bad). Those "flimsy" plastic buckles never broke.
While I have lots of respect for deco divers/cave divers/wreck penetration dives and even ice divers. I seriously doubt that they would have subjected the plastic buckles on the Transplate (or similar setup) to harder environs than the plastic buckles on my cheap US Army issued ruck sack.