The danger of using a PPA in an Ubuntu/ Mint install is more about the packages in the PPA being either bleeding-edge (ie. "testing" quality) or poorly coded. In order for someone to get a PPA they have to go through at least some (very) minimal vetting with Canonical which would dissuade most malicious parties. Any malware present on a PPA would get reported and removed in a hurry. Any software from a PPA that you've heard about through a reasonably popular web site has probably been used by enough folks; and the code gone over by enough pairs of eyes to satisfy one's concerns about the "safety" of the packages.
Having said that, many folks who have PPA's use them for working on their software, meaning that the packages may well be in development (read: unstable!) Some will have multiple PPA's for stable and testing versions of their packages. Using a PPA for something like an up-do-date build of LibreOffice or VLC or an NVIDIA driver is a safe bet. These packages will not have had Ubuntu or Mint's source code modifications made as they'd be coming directly from the (for example) LibreOffice or VLC or NVIDIA developers; but safety and stability are largely non-issues.
The official repositories, though, are the ones that are supported officially. Stick with those unless and until you have a pressing reason not to!