After around 10 days of playing the Metal Slug Defense v1.8.0, I have finally rescued all the POWs, unlocking the new units (Mummy Trevor, Elite Arabian Soldier, Cybord Allen Oneil, Morden Robot and Mars People Cadre) and the enhancement.
Seriously, I am impressed. Especially in the Emergency Areas, the enemies are getting much harder than before. The most challenging one would be the North Asia N. East. Out of the 5 stages, there 3 'Expert' levels and 1 'Hell' level. You would be probably facing some troubles before being able to defeat the stages with a Rank 'S'.
In this post, particularly, I would like to add some info regarding the POW rescue. Since the update of the Metal Slug Defense v1.5.0, you may discover that the POW saving is more likely a random process. And after completing those new areas in the v1.8.0, now I could probably verify this.