Well I make a test installing a odoo (branch 8.0 althouhg not the latest) and installing the product module with the builder and I couldn't reproduce that error (weird error indeed). It does't hurt if you post the complete stacktrace if you like or instead could you create an issue with it in github so we can check all that in one place please.
Documentation is one pending matter of the builder, but we made it quite similar to the odoo interface for the elements we manage such as models, fields, module itself, groups, record rules etc. Odoo developers shouln't be lost on the interface.
To highlight some of the current features:
- create module with basic info
- create of models, importing existing (models from installed modules) and manage external dependencies for imported modules, create fields, manage model methods, designer (web_diagram) for models (nodes) and fields (transitions for relation fields)
- create views for models and wizard supporting basic setting for most (except qweb) of the view types with custom code if needed
- creation of actions (currently only window actions)
- creation of menu items (can be mixed with current system menu if referencing parent)
- create groups (can be set on actions, menus, fields, etc)
- create model access rules with wizard to generate it for selected models and groups (generates ir.model.access.csv)
- create model rules (ir.rule)
- create cron jobs
- import local module (installed module) is a feature to inspect a given module (models, fields, relations, etc) is not intended for cloning a module and made modifications
- export and import module feature is not currently working right now
that's the features so far for the backend section, a website section is already created but not finished yet