Well the Ultramarines have close ties with their successors, so whilst they may not be quite as close knit as the Unforgiven they can bring significantly more allied Astartes to the equation. Take the Honour Company- the Ultramarines and it's dozens of allies keep a combined company permanently on station outside the Eye of Terror. That suggests that the UM coordinates with a significant number of Astartes.
If only they would Unite and launch the Second Great Crusade. I bet a few Space Marine Legions would give even the Tyranid hive fleets a good kicking!