• The 65C51 UART has a bug, with no intention of fixing it.
  It has no FIFO buffer, which is a problem with FTDI USB chips that send up to three bytes after the hardware handshake line tells them to stop.
• The Flash ROM is a PLCC. Most hobbyist programmers have DIP ZIF sockets, and will need an adaptor to handle a PLCC chip. And an extractor tool to get it out of PLCC sockets.
• The ready signal (RDY) is not on the expansion bus.
  I want to be able to slow down the CPU for chips that do not run at 14 MHz. RDY would need a flying wire.
• Flash ROM highest address bit is pulled high, and switched by Terbium software.
  I want to be able to replace the Terbium software with my own ROM code, without having to go through Terbium.
• The sockets are surface mount. This makes it much harder to modify, which I want to do for the previous points.

To attach wires to the circuit, one may have to take chips out of sockets to fine solder wires to the socket pads.