So I'm having the same issue. Had it with El Capitan, continues freezing constantly in Sierra. I'd use Safari for a few hours and it would become slower and slower. Getting the beachball thingy and can't move around in Safari. Strange thing is that moving between programs (always use my apps in full screen mode) still works. At least, it does for a while. Then, after another hour of usage (or even less) Safari freezes again, but the system completely locks up. With of course a force reboot as the only solution.
I've read most stuff in this topic and I think Apple should feel ashamed of theirselves. It's one thing to need some time in order to fix something. But it seems like it's such a widespread issue and it has been there for quite a while now. Again, as mentioned I've had it with El Cap as well. First time ever to have a Mac and then this happens. So sad.
It's not just that by the way. Occasionally elements on one or a few sites would freeze too. In that case Safari still works fine, and all buttons are clickable. But for example the hover animation doesn't work, until I close the tab and load the page again in a new one. Not shortly after that it happens again. Not always, but sometimes. Best solution in this situation is to completely restart Safari, which does solve the issue for a longer period. But it still occurs after a certain amount of usage.
I'm gonna try using STP as my main browser too now, because I'm really done with this. I have reported my issue with the complete freezings of Safari, but I did that earlier as well. Never saw any improvement. When Apple came up with Sierra one of my first thoughts was "Ah, they must've fixed those Safari issues for sure", but never happened as well...
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EDIT: I'm happy to report that this has really made a huge difference. Safari Technology Preview is more stable than I've thought. Haven't had situations where animations would get stuck, or where the whole browser would hang. Good to see. I hope Apple brings all of that to the normal browser very soon.