Tamlyn: Rachael Acks recently
posted on twitter that she sold a story after 22 consecutive rejections. It's not impossible!
Also, there's not much harm in trying. The 'cost' of submitting an existing story to a market is twofold: it prevents you from submitting that same story elsewhere until you get a response (assuming simultaneous submissions aren't allowed) and it prevents you from submitting another story to that same market until you get a response (assuming multiple submissions aren't allowed). If you have nothing else lined up for Uncanny, the second issue isn't a factor, and if you aren't sending any of those stories to other places, the first isn't a factor either. So I don't think there's any harm in sending one in.
What I might suggest is waiting until near the end of the submission period to see if any of the stories that you think are more viable come back from other venues. If so, then maybe submit one of those. But if not, I'd suggest submitting something anyways.
Edit: I suppose there's also a third cost--since most markets don't allow resubmissions, submitting now means you can't submit the same story to the same market later if you do revisions of some kind. So I guess it would depend if you're planning on doing revisions to the story at some later date.