While book value of equity per share is one factor that investors can use to determine whether a stock is undervalued, this metric should not be used by itself as it only presents a very limited view of the firm's situation. BVPS provides a snap shot of a firm's current situation, but considerations of the firm's future are not included.
For example, XYZ Corp, a widget producing company, may have a share price that is currently lower than its BVPS. This may not indicate that the XYZ is undervalued, because looking ahead, the growth opportunities for the company are vastly limited as fewer and fewer people are buying widgets.