How many failed nodes is acceptable

Ok you have a C8, now we need to know this

Also, your "Terrace Repeater" looks like it cannot find a route to the hub.
Try moving it closer to the hub (but in the same direction you wish to place it) and see if you can get it to respond to refresh / repair and get some better neighbors listed.

Overall, you have a pretty weak mesh, adding more z-wave devices would greatly help.