A lot of that uncertainty has to do with USPS, who keeps getting slower, and their delivery date estimates are at this point often 2-3 days more optimistic than reality.

At least in my experience, UPS, Fedex, etc., tend to deliver when the estimate says.

For me, FedEx is almost always a day or two over the initial estimate, and the package looks like it's been routed through a war zone.

UPS is generally pretty good.