I've found that mountain badass dinos seem to be less good at harvesting all the resources they are meant to, but more specifically one resource. For example in vanilla ark Ankylos are best at gathering berries (minus bronto), flint, metal and thatch. Sabertooths are best at Chitin, Keratin and Hide. Gigantopithicus' are best at Fiber and Wood. From what I've found with the mountain badass dinos so far: Sabers arent good at getting keratin and Gigantopithicus' simply cant gather wood. I would make an extensive list myself of the best dinos for gathering each resource but don't have the patience or the time.