True, but to give some credit to MS, they have deal with a much larger variety of systems, hardware as well as software. Plenty more room for something to go wrong. Looking at the iOS devices, there is only like maybe 15 different devices that support iOS 8, so more thorough testing can be expected.
It may look like that to the average individual. But it's not. Become a licensed OEM for Microsoft and find out.
Microsoft publishes a windows hardware compatibility list. That list is the hardware and components that they write for. And that's it. Guess what, it's not a long list of parts.
Now, that doesn't mean that all machines and parts in those machines are built based on that list. But windows only supports what is on that list. Anything else, and it's up to that hardware company to develop the software to enable that device or component to play nice with windows.
What Microsoft actually supports and writes for is a small list which would shrink the industry to a portion of its size if people followed it. Hence all the drivers and installation cd's that come with the things you buy or add on.
Yes, windows does come preloaded with some drivers. Guess what, those come from device and component manufacturers who want Microsoft to include them.
But, many main boards on the market wouldn't work at all in windows without first installing some patch from its manufacturer.
New computers come with windows installed and all the patches from the manufacturer installed as well.
Try installing windows from a retail package of windows and running it on a new machine without installing any patches or drivers and see how far you get. Go for it. Better hope everything inside that machine came from Microsoft's hardware compatibility list.