Zeballos can be dangerous when he's hot the serve looks awkward but can be really effective when he's hitting it. Seems to be having a david nalbandian season though, saving his best for davis cup having played big rolls in both of their underdog wins.
That's very true about his serve, his awkwardness helps him with it. He can get surprising pace and good direction with it from a serve position/angle that can confuse even us watching it, so just think about the players going against him. He can hit all serves with the same motion/ball toss, which is a plus. Also the left handed factor helps him. He's also got some variety and two decent ground strokes. With all of that said, nothing he does it at a consistent enough level or forceful enough regularly against ATP competition. He also doesn't move well enough or adapt when he isn't in offensive positions.
He has been big for Davis Cup, though, he's a very good doubles player.