The first ones were terrible. They seemed to have no good fallback if the building's gate was locked and nobody was available to buzz them in, and stuff just got lost. They were customer focused, yes, but that just meant they shipped me free replacement stuff with free overnight non-Amazon shipping. Not the best experience overall, though, compared to getting it on the original date.
Fast forward to 2016 and they're pretty good here. Though USPS has been best for my building (and elsewhere people here are talking about USPS being the worst - again, it's a hard problem to solve for every area for everyone).
* Package marked as delivered on Saturday. Package actually delivered on Sunday.
* Package not delivered because there was no safe place to leave it (there's a sign next to the front door with instructions do leave it in the complex's leasing office across the street).
* Package delivered to my door, despite my door being behind multiple card-access doors.
* Package left at the door to the leasing office, even though the leasing office was open and the lights were on.
These have all happened in the past three months. I'm borderline ready to cancel Prime.
At least Amazon is more than willing to work with you to figure out how they can make your experience more palatable.
UPS in the USA do [2]
[1] http://www.royalmail.com/personal/uk-delivery/special-delive...
[2] https://www.ups.com/content/us/en/shipping/time/service/next...
If you can void your guarantee based on "forces outside your control" in something like shipping, where you're at the mercy of traffic, weather conditions, and human error, it's not really a guarantee.
The UK is a substantially easier country to ship in logistically than the US. I don't doubt that Royal Mail's guaranteed delivery works.