This plugin saves me a lot of time when designing sites as it gives all the setting I need to create some nicely designed pages. I have yet to "hit the wall", which is what I call it when you want a plugin to do X and it doesn't.
I just updated one client's website to 3.9 and everything worked just as I designed it.
One thing I think needs to be improved are the instructions on where to put your custom templates as I was confused by how the installations instructions are worded. If you are confused about this, here is how you do it... In your theme folder (hopefully you are using a child theme) you create a new folder called "flexible-posts-widget", and you put your custom templates directly into that folder.