One thing I've been thinking about is that unless Akatsuki started with some Jinchuuriki people in from the beginning, it's likely they would never succeed in obtaining all of them. So far we have seen Naruto and Gaara, and they both happen to be very young and despite Gaara being the Kazekage, he was hardly amazingly experienced (and as we saw, the village of Sand had pretty poor general level of ninjas all in all). However, it's hard to believe all the people having such demons inside them would be so easily defeated.

Of course nothing indicates all the Bijuu would be already inside people - Kyuubi was free before Naruto - but if some happened to be inside some older badass ninja, the Akatsuki members sent to retrieve that one would more likely all end up dead. Maybe years later when some of them who have succeeded in obtaining theirs would go after such an experienced super ninja with their new powers they might succeed.