As Kim said, the apparent stiffness and toughness of Royalex varied a lot depending on the thickness of the sheet specified by the canoe manufacturer. But in the early days of Royalex in the 1970s, the sheets tended to be uniformly thick and the boats were very tough, but very heavy. As time went on, sheet was thinned in the forward and aft thirds to save weight and a few makers specified "ultra-light sheet which was very thin.
Uniroyal stopped producing Royalex in 2000 and thereafter the stuff was produced by Spartech corporation, until Spartech was acquired by Poly-One in 2013. Poly-One shut down Royalex production at the sole Warsaw, Indiana plant within a year. I have read that as many as 25% of sheets were rejected by some makers due to voids in the core. It is widely felt that around the year 2000 when Spartech acquired the Royalex production facility that the stiffness and strength of the material declined, due to the change in ownership and OHSA requirements which forced changes to the composition.
I have worked on dozens of Royalex canoes over the years and can say from personal experience that the thickness of the outer vinyl layers and the bond strength of the vinyl to the ABS strata declined rather dramatically toward the end of Royalex production. Also in my experience, Wenonah Royalex boats tended to use thinner Royalex sheet on average.