Flat panels yield far more output per area and per cost in bright sunshine, basic physics tells us that.
Common belief doesnt make something accurate. That belief comes from comparing systems rather than comparing equal collector cost or equal collector area. I'm sure you'll have noticed that tube and flat panel systems tend not to be comparable cost-wise or performance design-wise
FWIW old systems are always flat panel, and modern are mostly tube, and much more expensive systems too. Its no surprise that a higher cost system with better specs can produce more return, . but it would be unsound to conclude vac tube gives greater ROI than plate. It isnt that simple. We havent even started on the significant variation in flat plate designs and performance.
NT