That's true, plus the Wii sold a lot more than the GameCube so there's a lot more of them out there to break. Unless statistics are ever posted then there's no way to know which system has had more problems.
Still, one needn't look past the design of the Wii to see it's less durable. Compared to GameCube, the Wii has more components, more moving parts, and runs much hotter, all in a smaller space. You don't need performance tests to know that's less durable. Sure, it still beats the other systems, but that's only because it is a lot less technologically advanced.