It doesn't help that a lot of the players we do buy from South America are usually odd signings to say the least.
Sexy Lopez was never a blue chip prospect to begin with, Keirrison was either a bargaining chip to get Villa or a money laundering scheme, same with Henrique, Giovanni and Rochemback were signed during an era where everyone Barca was signing was crap. Our scouting in South America is not very good, though they did find a certain Lionel Messi.
Saviola was hardly a flop even if it ended badly, a disappointment perhaps but under Van Gaal he was one of the top scorers in the league and formed a nice partnership with Kluivert. During his first period with the club he scored 60 goals in 140 games, not elite but not a disaster either especially considering he was supporting Kluivert.
Riquelme was a poor fit who the coach didn't want. Bonano...well, that was a flop.