@Linkat,
It sure hasn't ever worked for Russell. There's a way to stab someone in the back (it's inevitable on a show like this) and still remain somewhat likeable, I think. Russell tended to rub their faces in it, though and ended up getting zero jury votes at both of his finals.
Sandra could be ruthless, too (which is why she was picked as a Villain instead of a Hero), but unlike Russell who tended to be sneaky and crass, she was upfront about it and won twice.
I'm not sure about Philip and his bragging about his oratorical prowess...as you said, he does have Rob worried about it, though. If the Zapateras on the jury are faced with three Omatepes in the final, they might just give it to Philip out of spite lol. Wouldn't that be something!