Custom form data can be sent to the Update Feed, but there is no way to report on it historically, or to pull then into a report.
ex: We have a custom form where our PM's enter a weekly status report. The form/update will show on the Update feed -but will eventually fall off & be unreportable.
Our users think because it is on the update feed, that I can always report on this, but I cannot. This is important data for us to have historically.