It might be useful to explain it in Buffy mechanics. Basically vampires in Buffy are super tough when it comes to blunt force trauma. Angel gets up from dropping off a skyscraper, they can be knocked hundreds of feet and get up just fine. But that applies to punches and kicks (as far as 90's special effects goes), and is completely different from something grabbing a limb and dismembering them (which happened with a vampire and another superstrong Buffy opponent, Adam, who had no problems whatsoever grabbing said vamp's head and pulling it off). As the ubervamp is specifically just a vamp (but uber), all of this still applies.
Sooo... if Drax just punches and kicks at a Buffy vampire without specifically going for dismembering, and barring racial weaknesses like wood to the heart, all he'll be doing is jack and squat. But if he actually tries to grab a limb and pull it apart, or reach inside (an admittedly reinforced) chest cavity and pull out the thing's heart, that would work.