The answer to the question is that Nihilus is a Galactic-level threat. Not in that he commands armies, but that he, himself, as a singular life form, is a galactic level threat. In terms of raw power, he is arguably one of the most powerful, if not the most powerful character in all of Star Wars. Other force users of the dark side who have achieved similar feats have only done so with long, drawn out rituals using multiple other force users as batteries and the like. It's arguable that Vitiate achieves a similar level of power when you are playing through Ziost, since he seems to wipe the planet out on a whim, rather than through a drawn-out ritual as before.
As for the Exile being the only one to defeat Nihilus because of being a similar wound: It's not bull. It is explicitly stated in-game that the reason Nihilus is weakened and defeatable is because he tries to feed on you, which he cannot do, and it weakens him. And as much as some folks would like to dismiss Nihilus, he was much more than even his hunger. In the cutscenes it is made very clear that he is also powerful in the force in a conventional manner. He literally strips Kreia of her power and nearly kills her on a whim. When Sion gets uppity (you know, the guy who is basically immortal until he CHOOSES to die) he literally mops the floor with him effortlessly.
Powerful force users in the conventional sense do not stand a chance against Nihilus because he can sense them, hungers for them, and feeds on them. For the record, Visas never RESISTED Nihilus' hunger technqiue. He spared her of his own choice.
There are many reasons why Kreia/Traya specifically sought out the Exile and not the least of which was because he/she was a wound in the force. There were other, philosophical reasons as well of course, but ultimately she needed someone who COULD destroy Nihilus, Sion and ultimately herself.
Now is that to say that Nihilus is truly undefeatable by conventional means? It's unclear. Kreia certainly couldn't go toe to toe with him on Malachor V, and she was ridiculously powerful in her own right. However, she was able to out-maneuver him and ultimately manipulate events that led to his defeat. A guy like Palpatine, also a master manipulator, with the appropriate knowledge, could likely arrange a similar defeat. But in a stand up fight, well, there aren't many that can stand before Nihilus without being crushed.