When we were really introduced to the Phalanx Clusters in FM: Warden Clans, what stuck out to me wasn't just that they had vehicles. It was that the vehicles were a pretty dominant force in the Cluster.
The 5th Phalanx had a lot of Asshur artillery spotters and some Hueys, and was known for its artillery barrages.
The 13th Phalanx was led by a veteran VTOL pilot and contained a full trinary of Donars, nearly all the Donars in the entire Clan. This gave the cluster impressive close-air support.
The 10th Phalanx was an unmixed all vehicle Cluster, led by the only tanker Galaxy Commander in the Clan. The unit was known for having a pretty capable officer corps.
So if there is any lingering differential between Phalanx Clusters and the other types, I wonder if it might be how central the vehicles are to the unit. It's more than "they have a single trinary of vehicles" in the Cluster where 'mechs/BA/ASFs are still the preference whenever possible. I think that might be the case in many other second-line/PGC type clusters. By comparison in the Phalanx Clusters, the vehicles are at the heart of the Cluster. They deploy a lot of them. The Cluster's preferred tactics are shaped by them to a greater extent than some other Cluster. The unit's traditions are rooted in being a vehicle-centric Cluster.