From the other side of the debate I'd attack the concept of subjective value.
Why do people value this? We don't know. They just do.
How do you know they value it? Because of their actions.
Why do they take those actions? Because of their valuations.
But it's also difficult to argue the counter-case. Outside of corner cases such as depression, psychotic episodes etc, if people aren't acting to diminish discomfort or increase their perceived happiness, then what is motivating them?
The issue comes when that extremely mangled definition of happiness isn't then injected into economic discourse - it becomes a matter of dusting off the hands and saying "well, we proved that people do what's required to be happy" and go back to the simpler definition.
In any case, reducing everyone to a stimulus-response box and stating that on balance, it must be beneficial to choose action X for the organism turns the definition of 'happiness' into something worthless and actually pretty banal.
Edit: I usually see this argument in the sense of 'maximising profit', but occasionally as 'happiness' or other beneficial emotion.
Economists, though, don't fuss as much about motives because they're not observable. I mean you can build a reasonably explanatory model by simply assuming people are greedy hyperrationalists. But nobody really does that because it's just too simplistic.
If I had a point, I guess it's that economics thinking is much more textured and subtle than people give it credit for. "Neoliberal" is a label that was invented by critics and so its meaning is basically "whichever strawman fits right now".
If we're naming names amongst the arch-neoliberals, I think these days I prefer Hayek to almost anyone else. I think he really spent time in the stew of ideas and didn't retreat from the revealed complexity of the world into equations.
How do you define "happiness"? What do you propose as an alternative to the subjective theory of value? (edit: if you disagree with it, I noticed it was the other guy hinting at being against it.)