Comparing consoles has always been difficult due to the vastly different achitectures that each console uses. The new generation of consoles, however, use very similar architecture making it easier than ever to compare their specs.
In the past, it’s been difficult to do truly apples-to-apples performance comparisons between game consoles because of the vastly different architectures of the various systems. You can get some raw numbers—clock speeds, memory bandwidth, FLOPS—and compare them that way, but how games looked and ran often had just as much to do with console-specific optimizations and tweaks from the developers as it did with the theoretical capabilities of the hardware.