Well, the ARM-1 is simple enough to understand [1], so the ARM Cortex might be comprehensible, depending on how much crazy stuff they added. But I suspect even at Intel nobody entirely understands the i7.
Bear in mind that 'Cortex' covers every modern ARM chip right the way from a tiny (~12,000 gates) microcontroller like the Cortex-M0 up to high performance 64 bit designs like the Cortex-A73, so it isn't a label that tells you anything about complexity...
You're right, I just checked out the older ARM Cortex, like M0 and such, and they don't look very complex. It probably get's messy once you have more advanced features like a cache, branch prediction, power saving etc. I really don't want to see the internals of a modern Intel/AMD CPU, or even an IBM one. Nobody can ever fully understand how it works starting at the lowest level, there are just too many features in there.
[1] http://www.righto.com/2015/12/reverse-engineering-arm1-ances...