I see Nuova Simonelli MAC listed more often than other brands in used market for 1 group commercial grade machine. Are they so good that more were purchased by commercial establishments or are they having problems and more users are trying to get rid of it?

"commercial grade" is different than commercial.

the machine is question is not one that most coffee professionals would consider "state of the art" (or perhaps even "desirable")

I thought NS MAC would be same caliber as La Cimbali Junior.

I picked up a NS Mac 2000v off of Ebay and its been working perfectly in my kitchen. Its plenty commercial and I would say far from undesirable. IIRC its a 2004 or 2005 vintage. It is a little on the large side, but if your kitchen can handle it, you would not be disappointed with it.