"Is the reason most deployments use specific load-balancers because of the administrative ease of using a specialized device with simple GUIs?"
Unsure I would agree "most deployments use specific load-balancers" because I've often seen Etherchannel or ECMP L3 being used. That aside, I would suspect something like a specialized device, with a simple GUI, would more likely to be used for its load balancing capabilities over just the fact that it has a simple GUI.
"Or, do well constructed networks configure load balancing in switches and routers in addition to the use of load-balancers?"
From my prior question's response, yes I believe switches and routers load balancing are used in addition to specialized load balancer or perhaps even in lieu of them or even without them. The major difference between a switch/router vs. a specialized load balancer (which generally does not go unnoticed) is switches and routers LB is "free" while a dedicated/specialized LB device is $$$.
"Do most specific load balancing devices also incorporate QoS?"
That I don't know, although I would expect some, if not all, would support some QoS, but QoS features can vary widely - there's no standard what "incorporating QoS" guarantees.
