I'm surprised how impractical the armor in fact is. In "Endgame", I perceived it as the ultimate defense, too.
Which surprises me, since even within the episode, the Borg adapted to the armor pretty quickly, so any advantage it gave was brief at best. Even in the first battle, before the Borg adapted, the armor was down to 40% integrity in less than a minute. It was never presented as "the ultimate defense," merely as a limited advantage that would delay their destruction long enough to run the Borg's gauntlet and get past it.