My experience has been that if they weren't issued immediately upon booking, they never come through. But the Plat. agents are telling me that "it looks good".
My experience is the exact opposite; In 10+ years as EXP, I've only had one SWU not clear. It was ORD-FCO maybe 5 years ago and the flight the day before had been cancelled so our flight was significantly oversold.
I fly primarily TATL, and usually book about 6 weeks in advance.
I do find that they clear later than they used to - but I'm still very happy w/my upgrade successs.