I usually create all my MetaBoxes through code but decided to give the builder a try.
First of all, great job overall it's extremely comfortable and faster than going through manual coding of all the fields. However there are a few shortcomings:
- There is no collapse all/expand all control in the fields list, making it hard to navigate through fields if you forget to collapse as you create them.
- There is no support for entering callbacks functions as needed (i.e. for default values or multiple choice options)
Then, I found a bug which is quite bad. I clicked on update, and then realised I had to change something else, then clicked update again. When the browser stopped loading all my field group settings and all the fields were gone. There are no posts revisions enabled so I can't recover them.
If you start from a group that has data and update before the fields complete loading, I'm afraid the system as it is will wipe all out. There should be some failsafe, or at least posts revision so we can restore a previous version.
I hope this helps improving the builder!