yup-that pretty much says it.
Either the Borg can only be used once or twice and retain their scariness, or they get used more and more, but inevitably become less scary, more run of the mill villains.
Actually, speaking of the Daleks, they actually did get weakened in the Current Dr. Who Show. The problem isn't that they get defeated everytime they are shown, even if they are shown every season/Series. The problem is when you Millions of them get defeated over and over again. In the Classic Doctor Who Show, you typically, only saw a relatively small group of Daleks (maybe 10 or fewer or 20, and maybe once or twice 50 or so), so it wasn't a problem to see them get defeated, because you always knew in the back of your mind, that they were only barely defeated, and if there were more of them, The Daleks would've won, so, that's how they remained a threat.
The Borg ran into the same problem, at first a single cube was capable of decimating the Federation, and finally Janeway was able to come out on top with her single ship, up against multiple Cubes (And of course, the Borg Queen didn't help matters)