Not really. You can get up to about 1Mhz on a breadboard, and my wire-wrap SBC works at 4Mhz without any issues. If you did run into timing issues, you could just underclock the whole thing.
That's true, but for a newbie like myself, it would be rather hard to get going.
Also, the current run of the W65C51 is majorly broken, which makes getting the project going even harder (most alternatives to the W65C51 are no longer sold).
Might as well build a Z80 system while I'm at it. And Zilog's UARTs are quite nice, or so I've heard.
I went with a 12V-EEPROM at the time, so I had to write my own emulator to speed up my development cycles :)