At the very least a party seeking equitable relief will struggle to meet the test of "clean hands" which the courts require.
Lack of a prosecution does not mean a lack of legal consequences.
Although in exceptional cases where third party rights are not affected the courts might be persuaded to treat the stated date as being the effective date, a situation we return to below. There are rare occasions when it may be permissible or even justified to do so.
Although criminal prosecution might be a risk in serious fraud cases, in most day to day legal matters where backdating occurs for reasons of administrative convenience, or simply by oversight or error, the risk of being charged with a crime are commensurately small.
But even if a person is not charged with a crime, the fact that a crime can be demonstrated to have occurred may still impact the rights of the parties.
Legally speaking, this is something that you should not do or more accurately, there will only ever rarely be occasions when this is appropriate to do.
However in practice, for both good reasons and bad, backdating of documents does occur.