Yes, we currently convert the Unity Triplanar node into the MaterialX Triplanar Projection node, and it has fewer options than the Unity equivalent (including a lack of the “Blend” input). We have it on our list to reimplement the Triplanar node ourselves (as we do for nodes that have no MaterialX equivalents), but haven’t gotten around to it yet. I will bump that up on the list so that we get it out in a future release.
In the meantime, the only workaround I know of would be to reimplement the Triplanar node using other nodes based on its reference code.