Or maybe Tsang just doesn't have what it takes.
I think he has what it takes, I mean if you set aside accurracy and judget the sheer detail and quality of sculpture on the HT Alien figures, they're beautiful works.
The realy question is, why do they depart from the source material. There is no source material in existence to come up with the design of anatomy used on the ALIENS figure, it's completely conceptual. Hot Toys made a snap kit ALIENS figure that was a great job, for a 4" figure captured as much Giger design as possible, but then at 1/6, it wasn't even in the ballpark.
I really think there's some sort of intentional reason for some of the innaccurracies, a stylistic choice or something, they just seem too deliberate and not due to error or inability of sculptors.