I have been using my Myki card for well over one week without any problems so far, except one stand alone balance reader would not read my card. I have not as yet used it on a bus, even though I often use buses. That may be a good test for it.
Originally I had the card with other cards and when I attempted to scan my card while it was still in my wallet, I received a message along the lines of 'Too many cards'. I moved the card so that it was on the other side of my wallet and on its own and it scans fine through the wallet.
The scan time seems very variable though, up to a second at times, but not always. It certainly is not instant. Other countries seem to have readers that are instant. Given the money spent on the system, I am surprised that you can't just walk past a scanner with your Myki card in your hand and it is read. The system is smart enough to know that if your card in your wallet on a crush load tram is pressed up against a reader, that it is not multiple trips.
While there will be problems in the early stages, as there was with Metcard, I think Myki will settle in time and future proof the ticket system for a decade or more. But gee, it has cost an awful lot of money that could have been spent elsewhere.