According to a response from Phasedragon on the official discord, putting a UdonBehaviour with None syncmode on a gameobject will break syncing for any other UBs on that gameobject.
When building reusable components, it would be preferable to be able to slap None mode onto any UdonBehaviour that does not, itself, require sync, while still being able to add other UdonBehaviours that need sync. Consider, for example, a simple UdonBehaviour that adds collision sounds to any rigidbody. We should be able to add this regardless of whether the object is synced or not. With the proposed None sync mode behavior, it would be necessary to mark this helper UB as being either continuous or manual sync mode (and it would only be able to coexist with UB of the same type).
If, instead, none sync mode was considered 'weaker' than ObjectSync, Continuous, or Manual modes, and to defer to those modes if a corresponding component was present, we'd have much better composability than the proposed semantics.