Aunque sea para cometidos diferentes se me hace extraño el que no se hayan decidido por un único diseño, aunque sea en diferentes versiones (como los BAM españoles).
Y sí, las capacidades son diferentes, pero vaya...
a concept of Platform supply vessel,[1] or Anchor handling tug supply vessel.[2]
[...]
they feature a relatively low maximum speed of 12 to 15 knots, but are able to operate for 30 days without resupply
[...]
They carry an 8-metre boat and several rigid-hulled inflatable boats,[16] and can deploy frogmen as well as Remotely operated underwater vehicles.[1] The ship can also accommodate a medical team to be deployed during humanitarian emergencies.[5]
To assist ships of distress, the B2M can develop a 30-tonne bollard pull and have extensive anti-fire capabilities.[1] They furthermore feature a crane capable of lifting 12 tonnes with a 14-metre reach, or 10 tonnes at 17 metres.[3] This allows autonomous loading and unloading of standard containers,[16] a useful capability for humanitarian operations in disaster areas.
Y para el BSAH:
The  diversity  of  missions  led  to  the  proposal  of  a Supply  Vessel-type  ship  with  a  towing capacity  of  80  tons,  a  speed  of  14  knots  and  significant  autonomy  of  about  30  days  of operation without refuelling. The vessels will be equipped with an 8-metre working boat and semi-rigid  boats  as  well  as  a  crane  allowing  the  embarkation  and  disembarkation  of containers, and can also accommodate divers, transport weapons and ammunition, provide support to a submarine during stopover, and deploy an anti-pollution barrier. 
Aunque bueno, mirando el listado de 
https://en.wikipedia.org/wiki/List_of_a ... Navy_ships se aprecia que, por ejemplo, también tienen bastante variedad de remolcadores oceánicos, así que tampoco parece extraño el que no se busque la máxima homogeneidad...
Aprovecho para indicar que en esta lista de la wiki se indica que la clase L'Audacieuse será reemplazada por nuevos patrulleros ligeros... en 
http://www.navyrecognition.com/index.ph ... -area.html hay más detalles al respecto.