Here's my thoughts on this:

Pretty much, just treat it the same as this meta post ([What is the policy concerning external deb packages?][1]), and as this one ([Is there any guarantee that software from launchpad PPAs is free from viruses and backdoor threats?][2]).



In summary, here are my thoughts on this:

- Warnings. Something like this might be wise:
    
 > Warning: This involves install software from a non-Ubuntu repository. Please, be careful. For more information, check out [this post][1], as well as [this post][2]. Thanks.

- As someone writing an answer, try *not* to use a external app store if possible, but at the same time, don't be afraid to do it if there is no other way.


  [1]: http://meta.askubuntu.com/questions/2364/what-is-the-policy-concerning-external-deb-
  [2]: http://askubuntu.com/questions/7662/is-there-any-guarantee-that-software-from-launchpad-ppas-is-free-from-viruses-an