Having the same problem (can't modify cell borders in Numbers for iCloud beta) I told them via the feedback function already 6 weeks ago. Since then I open Numbers for iCloud once a week to see how it evolves but it does not seem to do so at all.
The actual iCloud version has less functionality than Numbers for iOS (which is already very restricted compared to Numbers 09 for OSX).
I could live with that (it's a beta version) if I saw at least from time to time an improvement, especially since it is a web app which can be updated at every moment because it's reloaded every time you start it.
But for the moment there is only stagnation on their side and frustration on mine.
Did the developers read the book "How to kill the early adopter's enthusiasm" ?