My unsupported theory is that which carrier is best depends on where you are—that would explain why different users love or hate different carriers.
In San Francisco, Fed Ex is the least reliable. I have seen their site actually show a package as on the truck for delivery for more than one day. Sometimes they are fine, but you can’t count on them.
UPS here is generally good in my experience.
If I can, I will ask for the option to divert the package to a UPS/Fed Ex store near me for pickup. Then if the date starts to be pushed back, I am not caught staying home over and over to meet the delivery.