Jonathan, not sure it was the hull flexing that caused the tubes to fall off. BWM had some quality issues are yours are not the first we have seen fall off. However the hulls are good if a little basic do stay together suprisingly well.
Do hulls flex, yes. Modern ribs have quite high decks so you can't see much of the hull. However we crossed the North Channel between Larne and Stranraer in a 6m early humber with a very low deck. At 20 knots you could see the hull flex as we bounced through some big waves. However that hull went on for years without any problems.
Nice boat but just glad I am not paying the fuel bill for those Yamahas