I've heard a lot of people talk about using this as a front end for their own rom collection. I looked at a few YT vids and it looks legit. However on the Google Play store it specifically sais do not install this on adopted storage (something about it deleting UserData on Android; a bug) thing is I'm using a USB thumb drive adopted as internal storage, is it safe to download this or should I wait until they come out with a fix for this :/ and more importantly will they ? It's a shame, if I can't use this are they're any alternatives preferably with a decent UI ?
Gamesome or Nostalgia are other options. I have been using Arc Browser and love it. I have my games on a portable USB HDD and the data directory on my portable microSD card.
I have not tried installing on Adopted, but if that is your only option.. I would not expect much. You could email the developer (see email in the Play Store). Kim is good at responding.