Is it difficult to do high voltage parallel programming using Nick Gammon library?
I have not tried with those instructions, as I have an AVRDragon, but it's a good exercise for precision, since you have to wire lots of pins.

20+ iirc

My guess is that if anything fails, it's likely to be because of a mistake in the wiring.

If you are planning to do it often, you might consider soldering the wires and the ZIF socket to a prototype board, to preserve your mental sanity.