I've definitely had issues doing an upgrade on a real Ubuntu installation. Years ago I needed to update a more or less clean install of Ubuntu Desktop. It managed to completely break Gnome somehow (This was before Unity). I'm guessing things are better now and that the server distribution is better for this but I could see complications happening.
The 14.04 version which is supported until 2019 still has the bug that it won't clear out old kernels after upgrading them. So if you don't look into your /boot partition now and then it fills up and breaks your installation.
Xe could hyperlink you to Dustin Kirkland's call for feedback here on Hacker News only a short while ago, where this subject came up several times. This is a well-known Ubuntu problem, and M. Kirkland xyrself said that taking purge-old-kernels out of the byobu package and putting it somewhere more obvious was a good idea.
Of course, this is an Ubuntu upgrade problem relating to Linux. It is specifically about kernel image files. So it has very little application to the Windows NT Linux Subsystem, which as we know uses the Windows NT kernel.
After a major change of desktop like a switch from gnome 2 to unity or gnome 3, it is not surprising that users may need to refresh their configuration files. I am doing dist-upgrade since 2009. They were all smooth except the upgrade to 16.04 (lost of network and a couple of small issues still not fixed).
After 15 years with Ubuntu, I've only ever seen problems when people switched default Ubuntu packages with their own (back in the day it was often beryl, compiz or drivers).
I have actual machines that I was able to flawlessly upgrade from 4.10 to 17.04. Can't say that about Windows. Though I did actually know what I shouldn't mess with if I want the upgrade to go through (or fix before).
I hit problems with the desktop environment on both 8->10 and 10->12, then gave up trying to upgrade Ubuntu in-place. Debian has been absolutely fine, though.
> Because you can upgrade from windows 3.11 all the way to 10
That has never ever worked for me until the 7 -> 10 upgrade: 95 -> 98, 98 -> 2k, 2k -> XP and XP -> 7 all failed miserably and required installing from scratch.
And even 7 -> 10 failed without any helpful information= the first few times I tried it (unlike OSX where I've been upgrading and migrating the same system for more than 10 years across multiple machines and versions).
Wondering why such a disclaimer? Ubuntu apt upgrades are reliable. Or is it related to WSL? Anyway good job WSL team.