I realise this is basically a "homebuilt" thread, but FWIW one needs to consider aircraft downtime resulting from avionics failures.
Managing individual avionics failures is much easier than managing a collective failure. With individual instruments, you can swap out the duff one.
This is why I like individual instruments. In most cases, the swap is trivially easy to do, a signature (on a certified aircraft) is obtained, and the downtime is only ~ 2 days even if the replacement had to be ordered from the USA. Whereas with a "glass" cockpit, most problems involve a flight to the dealer, trains to get back or hotel stays, etc.
Maybe, with cheap enough (homebuilt / uncertified) kit, one could keep a whole spare multifunction unit on the shelf?