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?), and as this one (Is there any guarantee that software from launchpad PPAs is free from viruses and backdoor threats?).
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, as well as this post. 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.
