I wonder how would it compare to an old 8 bit CPU like 6502 or Z80.

It is vastly underpowered and has a huge bill of materials compared to those chips. It only supports 16 bytes of memory, has an impoverished instruction set, is slow as molasses, and tends to be very flaky.

Even as a pure-ttl building exercise, it trades off efficiency and usefulness in favor of understandability and fancy blinking-lights so you can see what is happening.

It is useful only for toy programs at the most.

But as a learning exercise it is absolutely first rate. 10/10 would recommend.