At least, that's what it seems like to me as a layman.
The annoying thing with Amazon shipping is that sometimes you can't order an expedited shipping option if needed, even for prime items. This tends to be due to third-party sellers that participate in the prime program, but for some reason don't offer any other option but the free 2-day. There have been times when I've paid the extra for next-day air for some things, though it tends to be very expensive.
The larger issue is that I simply no trust Amazon to actually deliver. Between Amazon "accidentally" switching my purchases to something non-Prime and not actually having something in stock in spite of their claims to the contrary, I cannot count on any delivery date promised by Amazon.
If I need computer parts, for example, I'm going to Newegg because they will actually have the part and actually deliver when they say they will.