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I've done some dead simple forwarding/load balancing work, and if you can do it with nat instead of a proxy application it'll use a lot less memory, in addition to less cpu.

That means fewer load balancers needed, or smaller machines (or both). So I'd say that means anytime you run out of capacity on your proxy machines would be an opportunity to look for other techniques. Haproxy is probably easier to use though, and would tend to need less work to get the features you want, though. So there's an opex/capex vs development time argument.

Hyperscaling Haproxy is a lot of fun too, though. There's a huge difference in connections/second between a normal config and a totally tuned config with haproxy and kernel patching on the table.