What we get from Qu Rahn is that Jerec could have become a star-system level threat with just a thought or a whisper. Considering there are billions of star systems in the galaxy, he would have to output as much energy as billions of thoughts require in order to destroy the galaxy the way Vitiate almost did. And so, it stacks up to the point where he wouldn't be the kind of threat that the galaxy hadn't seen before. And The Terror From Beyond was also a galaxy-level threat, if left lose for long enough. I wouldn't say that Jerec, even if he'd succeeded wouldn't be that much more powerful than certain force entities like the Wutzek.