Hi!
As we know there are no open issues with Mach and PoKeys. They works stable and reliable. Second serial port may be option, but with PoKeys you will get much more. E.g. a lot of aditional keys, LCD, lot of outputs etc. If you need aditional relays consider also PoExtBusRe. You can add up to 80 relays using only 3 pins on PoKeys.