I've not rated Walsh at all the last couple of times I've seen him - nothing major but lots of petty fouls given and some more obvious ones left as well as fairly random decisions to book.
RObertson is probably my favourite ref in Scotland because he lets a game breathe but I think sometimes his eagerness to do that means he doesn't give some fouls so as much as I like him and would happily have him referee Motherwell every week, I think the other three are absolutely consistent when giving fouls and cautions and can handle the big games.
Also neither Walsh or RObertson have the big game experience to do a final - neither have done an Edinburgh or Glasgow derby, Robertson has just had his first go at Rangers v Aberdeen and made one KMI error at least, possibly two. I think they could both make it to Final Day but they're just not there yet.