  Different approaches have been applied to the calculation of form factors of
various hadronic systems within relativistic quantum mechanics. In a one-body
current approximation, they can lead to results evidencing large discrepancies.
Looking for an explanation of this spreading, it is shown that, for the largest
part, these discrepancies can be related to a violation of Poincar\'e
space-time translation invariance. Beyond energy-momentum conservation, which
is generally assumed, fulfilling this symmetry implies specific relations that
are generally ignored. Their relevance within the present context is discussed
in detail both to explain the differences between predictions and to remove
them.
