I've got to be honest, I have mixed feelings on PPA's. I can see why they would be good as it allows the community to essentially connect with individual repositories in order to download an application developed by a sole dev or team of devs. This allows for the application to be maintained separately instead of relying on the community as a whole (granted there is enough interest) to maintain the software. However, it has it's drawbacks. What would have been its strength in that it is a separate repository maintained by a team of devs is also its weakness. Typically, what ends up happening is that the developer(s) either slow down on development which may make the application or package incompatible with newer software (which is being released all the time), therefore making the application unreliable or flat out not work. Alternatively, the devs lose interest in the project and decide to abandon it altogether which brings me to my next point.
Unless you do some digging around to see whether a project is still active or not, you may not know at first glance whether or not a repository is still being maintained until you decide to attach it and try to install "x" application to find that it is missing dependencies or crashes only to find at a later time that it was discontinued. You don't see this on other distros such as Manjaro because the software is essentially maintained actively by the community or in-house by the Manjaro team. Same can be said for the Arch User Repository which essentially brings together what would have been in a number of different PPA's into a single library which is by far more convenient for the community and allow for ease of access and installation. Essentially, that is another gripe when it comes to PPA's is that they would have to be individually added in order to have access to different software, whereas all you would have to do from the standpoint of installing software from the AUR is to run Yaourt and basically download/buildwhatever software you are after all in one go instead of adding PPA's and then having to refresh repo list, then try to download and install "x" application.
I'm also sure there are security concerns with them as there are less visibility on individual PPA's which in turn makes it difficult to verify whether or not the software contained within "X" PPA is safe or malicious (granted there aren't many malicious programs on Linux), but they still exist. You would be surprised.