[fen ""] 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5?!
Why is ...e5 considered dubious for Black here, although it is considered sort of a refutation after the move 5.f3?! e5!
What's the big difference? If it is strong after 5.f3, it shouldn't be so bad after 5.Nc3, the pawn structure is the same.