.....who is of the opinion that yes, Apple can make computers that 'just work' out of the box when you have just a limited and tightly controlled range of hardware to support. Not that difficult.

Easy streets, yet I'm amazed at how often they seem to cock that up.

Whereas MS gets the flack for making software that has to work with billions of combinations of kit of all varying degrees of quality. Yet on the whole manages to get it to work pretty well.

Which is the greatest achievment/technical challenge?

It's no wonder Apple never lets OSX out into the wild open. It would die a death out there.