I’ve read that Ubuntu is a controversial OS. Why is that?
As a member of a LUG in the mid 2000s, when ubuntu was first released, it became all the rage in my neck of the woods. Senior members of the LUG were promoting it. In the technical open source press it was winning praise and some awards. The LUG had a few debian developers as members and was a deeply debian environment but was impressed by ubuntu. Many members began using the new distro and were very pleased with it. It was new, modern, up to date, fully-featured, user-friendly and appeared no less sophisticated than other distros at deeper levels. Ubuntu was actually initiated by Canonical engaging a number of debian developers who were gathered by the company to create the new distro, so the debian pedigree was clear. Over the following years though, a number of things happened with ubuntu which sullied its initial appeal and praises.
The corporate nature of ubuntu had an effect. Ubuntu is not a uniquely open source community distro, though it uses a great bulk of free and open source software (FOSS). The corporate control of the distro means that it has agendas commensurate with corporate and commercial objectives, in particular, monetisation. This can conflict with the sort of ideals of research that is freely shared for the betterment of all. The corporate control aspects contrast with community distros like debian which is organised in a democratic arrangement, and contrasts with other community distros that are lead by "benevolent dictators" like slackware, or small independent groups of developers whose work honours the FOSS ideals.
Around 2012 ubuntu shot itself in the foot by its use of telemetry that was rather offensive to FOSS users who really expected to be able make those sorts of decisions about what their systems would send back to outside agencies themselves. Ubuntu included a "shopping" feature which sent user's search queries to amazon. Privacy was breached and trust was eroded somewhat for the distro which was called "spyware" in some circles. Canonical withdrew the feature after significant criticism from the Free Software Foundation, Richard Stallman and others.
Over the years ubuntu made a number of software decisions, often abruptly, that were not well received by a number of users. They initially used gnome for the desktop environment (DE), but then abandoned it for a home grown DE called Unity which they hoped would be good for desktops, phones and tablets. After criticisms, they reverted to a later version of gnome. They did a similar change from X11 to what they called Mir which was a new display server, but abandoned it after some difficulties with replacing all of X11. They've embraced wayland currently.
Ubuntu recently has begun developing a replacement for the coreutils that ship with most linux distros and include the basics like
cp,
ls,
rm,
echo,
chmod,
chown etc. It's called: uutils. Canonical want to create a complete drop-in replacement of coreutils, with uutils. The project in itself is not in question so much as the strategy that Canonical appears to be involved in. The uutils software comes with the MIT licence instead of a GPL licence. That means that the code can be used and enhanced without needing to supply any new code back to the linux community, which contrasts with the GPL that requires the supply of new code that is used to enhance the software and is publicly distributed. Effectively Canonical create the opportunity for themselves to use the developed software with their own enhancements for their commercial objectives without contributing those enhancements back to the FOSS world of their origins. Some users see this as bad faith in the FOSS community.
Ubuntu's snap packages are not well-received in some FOSS quarters. Whilst being software with some advantages, they are ultimately controlled in an exclusive centralised store run by Canonical which is not a FOSS depot.
@gvisoc explains the detail in post #5.
The advantage of a distro like ubuntu with its commercial arrangements is that users can buy official support which is attractive for commercial operations, small or large. Canonical has made positive contributions to linux overall and still does. Originally it was user friendly in a way lots of other distros weren't with the way its GUI worked. Canonical also shipped ubuntu disks free all over the world, which explains how so many users here got them. That was an influential initiative. Canonical's development of the lxd and lxc container technology has been significant, but there are other developments too. Today I'm a bit out of date really with ubuntu so I can't say more.