There is little that can go wrong in a clean installation, but much
room for error in an upgrade --- and that goes for any OS. Many distros such
as Fedora do not even encourage it.

With a clean installation the root partition is reformatted, so you are
starting with a clean slate. So unless your burn or download is bad then you
have few problems in comparison. When you do not reformat, as in an upgrade,
there are traces that inevitably get left behind and these can come back to
bite you. Also there are changes from one version to another that make it
necessary to convert various parts. For example moving from grub legacy to
grub2 or when xorg.conf was done away with or Plymouth replaced usplash.
These are all room for problems that would not be there if you did a fresh
installation in the first place.

My system is so customised that an upgrade usually fails anyway. However,
many years ago, I set my partitioning up so that I had a separate home
partition and I still have my original home folder, even after I went from
ext2 to ext3 and then ext4. I have not lost one piece of data after all this
time. In fact I have to manually go back and delete hidden folders for apps
and settings that I no longer use or need. I have even switched distros
several times without a hitch.

If I could pass one one bit of advice it would be that every user should do
themselves a favour and take the time to set up a separate home partition.
It has saved me countless headaches. The second piece of advice would be to
always do a fresh installation. Most people who have been around the block a
few times do clean installations and they know how to bail themselves out of
problems that come through upgrading. It is just good practice.