They do still have the most total of all alliances. I've fought a number of times both with and against NPO nations, and they are well above average in terms of both military competence and preparation. Well, most of them.
Molon Labe was maybe better to fight with, but that was a long time ago, and I was still pretty green at warfare when they were teaching me what to do. Nueva Vida seemed like really tough opponents, but that was almost as long ago that I fought them.
Ah, Orion. I remember rolling (an alliance named Orion) in defence of NPO way back when. Not the toughest alliance I've ever fought; maybe they'd have survived though if they'd stuck with their leader, he seemed like a good guy.