I am not really in a position to say. There are a few projects (like Busy.org and eSteem) where I trust the developers themselves, but it is impossible to know how good of a job they are doing with their security.

Generally sharing your posting key with those apps isn't super risky, because they cannot take your funds - but someone can still do a lot of damage with your posting key..

Steemit, Inc. is planning to add support for third party apps via a tool that they will host which will manage users keys. Once this is done, this will be the 'officially supported' method for third party apps to handle users' keys.