This CAN be true, but you needn't worry about some sources. Medibuntu for
example is even recommended by Canonical. Getdeb can cause a few problems as
can some PPAs, but this is more due to the fact that these sources create
problems because some have different versions from the ones that Ubuntu
stocks and can lead to breaking the package manager. It is usually a simple
fix, but for a newbie it can seem like a lot. Just typing will usually do
the trick sudo apt-get -f install.

I don't want people to feel that they are limited to sources provided out of
the box. You just have to not over do things. Start small and branch out
from there. There are lots of great PPAs and other sources for packages. If
you are a photographer and want GIMP 2.7 then you need to use a source other
than Ubuntu's which only gives you 2.6 or if you want a different version of
OpenOffice, for example. the sky is the limit.

I was more concerned about people bungling their systems by trying to
download a package from who knows where or trying to compile and ending up
with a mess that they cannot get out of. Package managers track and are able
to remove. If you do an end run on the package manager and do not know where
you put something then you may be stuck with the mess as it will have no
record. So far Linux has not been able to overcome human stupidity. They're
working on it, but we are inventive creatures and find new ways to get into
messes.