I personally don’t use this particular app and I don’t know why it’s not included in the main repo.
The policy for inclusion in the main f-droid repo is very strict and in the izzyondroid repo you’ll usually find apps that don’t fully comply with the features required for inclusion in the main repo either because they wouldn’t work or because the developers aren’t interested in making the necessary changes or because the developers are actually making the necessary changes but the process hasn’t been completed yet.
Personally I consider the izzyondroid repo to be reliable as the mantainer is @Izzy who is an active f-droid contributor and also works hard to help developers so that their apps can be published in the main repo. Obviously it’s always good to pay a little more attention when installing applications from repos other than the main one.
Here you can find a thread about the known repositories (where you could look or ask for informations): Known Repositories - Wiki - F-Droid Forum – Known Repositories