The featured image does indeed work. I think the problem stems from people that may want the featured image as something like a required field, or at least prominently displayed.
As it is now, it’s somewhat of a hidden feature. This is due to WordPress design, but the backend does have it both in the Media Upload and a separate field — thus easier to spot and add.
This plugin does it fairly nicely by adding the featured image to the top of the post form: https://www.ads-software.com/plugins/wp-user-frontend/
I’ve been trying a few different plugins with front-end posting and it seems that the very few that are out there are good but all of them seem a bit incomplete (of course yours is very new and still in beta, so that’s a given).
My favorite so far, out of the box without any editing, has been Front-end Editor: https://www.ads-software.com/plugins/front-end-editor/
The way it handles editing of current articles is great, but it has less functionality than the one posted above: no guest posting; awkward way of implementing front-end posting; no way of viewing your currently pending posts (if you’re on an account without access to wp-admin and want to see it on the front end), etc.
Anyway, sorry for the lengthy post. I’ve been enjoying your plugin so far and I’ll keep an eye with your progress. There’s a lot of potential with this one!