I have wrestled with this same problem for four days. Went through all the options to rebuild the library, but always froze at about 90%. Had telephone support help, but all options exhausted, so went back to basics myself.
I trashed the new version of Iphoto and went back to the old one. I opened my Pictures folder , right clicked, and got it show originals.
I then went through the originals year by year, folder by folder, picture by picture (6,800) and dragged onto the desktop any folder that contained OTHER THAN jpgs. (psd Photoshop format in particular.)
I reinstalled the new Iphoto and it worked. Everything intact.
It clearly didnt like handling other formats, though it was happy with png.
OK I have a fair few folders on my desktop which I will have to go through and change the psd ones to jpgs, but it works, so Im happy to do that over a few days.