This unfinished post hails from 2013. Is any of it still true? No idea.
accepts_nested_attributes_for (use view models)
instance variables in the application controller – every controller and every view can see it. There’s no great place to put an object that lives for the whole request (in the request? in the response? in an object temporarily in the config?) People like Mongoid’s IdentityMap end up using static methods with thread locals for thread safety… that’s not a great way to do it.
Error messages put together in models, and i18n as the only alternative – Many people construct error messages directly in the model. In the best case, they may use the built-in i18n support to provide parameters to be substituted into a phrase. However, the model is still responsible for formatting the objects as appropriate in order to be substituted into the i18n string. If, for example, you need to include a list of objects in the error which should then be formatted to a more complex string (for example, a link to the article), it is not easy to make use of a view or helper to perform that functionality for you.