Shenan1982: Its a beta. I know thats become a typical answer to people who are concerned about the stability of certain aspects of 10.10, but its the truth. We just can't expect a beta to be the final product - or even close to it.
As you mention, they are rebuilding/making changes to many (if not most) apps running inside OSX, not to mention the actual OS itself. The stability of Safari might not even have anything to do with Safari, but might boil down to instabilities in other running processes. Just as expected with a beta or anything else under progress.
I too have had some problems with the beach ball, but after disabling flash and telling Safari to run certain pages in "Safari 7-mode", its been pretty good. Got some problems with the links on the favorite-bar disappearing, but thats the only issue I got with Safari right now.