Here is what NETGEAR had to say about the all-clients-on-one-radio problem:
cc666: They were going to contact you to follow up. But it appears you already returned the router.
SmartConnect should have balanced the load by associating the 4th client to the second 5GHz band.
In our test we found some WiFi devices do not retry to connect if once refused by the router. In such cases we allow the device to connect to whatever band they like. It seems the customer is having one or two such devices.
cc666: They were going to contact you to follow up. But it appears you already returned the router.