Form factors in relativistic quantum mechanics approaches and space-time translation invariance

Invariance of form factors under Lorentz boosts is a criterion often advocated to determine whether their estimate in a RQM framework is reliable. It is shown that verifying relations stemming from covariance properties under space-time translations could be a more important criterion. Form factors calculated in different approaches for a simple system are discussed with these various respects. An approximate method is shown to remove the main discrepancies related to a violation of the above relations.