Buendia is completely useless against any remotely physical/intense opposition. Unfortunately, that is at least 3/4 of the Premier League.
I don't think it's physicality that he struggles with. He gets stuck in WAY more than you'd expect for someone his size, and often wins aerial duels he has no right to win. I think his issue comes down to pace. I mean his top speed and his speed over the first couple of yards, both of which are below average. Maybe it's what you might call 'intensity', but when the game moves quickly (up and down), he struggles. If he's leading a break, he's likely to be caught by a quicker player, if he's chasing back, he's unlikely to catch anyone. If the game is settled in one area of the pitch, then he's fine.
Ironically, if Forest do park the bus next week, he's likely to be more effective against a team playing that way, than one willing to attack us at pace regularly.