The Sleepless presumably do not want her to swear a Radiant oath because she would be able to use the Dawnshard in conjunction with Surgebinding, and we know that that combination already destroyed one planet in the system so it's pretty understandable.
But there were a bunch of Soulcasters lying around and they didn't seem bothered. So is this one of the differences between Radiant Soulcasting and Soulcasting via the fabrial? That the Dawnshard cannot be used alongside the fabrial?
So, the Sleepless ARE capable of Radiant bonds. (I believe the back jacket of the first book implies as much, if I remember correctly.) However, things they at first thought were great are making them increasingly worried, for reasons that will come up (not related to them specifically) in this book and the next.
Soulcasting via a fabrial is way, way less dangerous than Radiant Soulcasting--which is in turn far less dangerous than unbound Soulcasting (meaning without oaths.)
We've seen that the interpretation of the oaths is largely up to each individual spren (to the point that we've seen an entire order of radiants change their allegiance). Would it be possible for there to be a "sociopathic spren" that has interpreted the oaths so radically differently from the rest of their kind that it appears, to an outsider, that they are unbound in the same way the wielder of an honor blade is unbound? Or is there something essential about the nature of spren that prevents this?
I think that spren could go further than we've seen so far, and indeed, many of the older Skybreakers might be horrified by how far their order has gone. However, there are SOME fundamentals that even a spren with a very different interpretation wouldn't be able to abandon.