I finally got a chance to dig around in the form builder recently, and I agree that it is not completely intuitive to use. It took me quite a while to just figure out how to reduce the height of each text field.
One thing I noticed is that there is no way to specify the field name (and id). It appears to be auto generated by the text you enter for the label content, along with the form type.
This causes several problems. One is that if I use the page to do something like send an email, or insert into a database, and I change the form name later, it seems to change all the field names, thus making the variables I used in the database insert, or email not work any more.
Also, it makes many autofill (like google autofill) not work, since they are looking for common field names like first_name and last_name, email, phone etc.
It also can make extremely long field name, and class names, which make the code unwieldy.
Or perhaps I just missed something somewhere.