You make it sound as if there is some signal or register in the brain whose value represents "pleasure" in a straightforward way. To me it seems much more plausible that "pleasure" reduces to a multitude of variables that can't be aggregated into a single-number index except through some arbitrary convention. This seems to me likely even within a single human mind, let alone when different minds (especially of different species) are compared.

That said, I do agree that the foundation of pure hedonic utilitarianism is not as obviously flawed as that of preference utilitarianism. The main problem I see with it is that it implies wireheading as the optimal outcome.