  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.
