Fascist and fascist adjacent nationalism tends to define the “nation” in a way that excludes and endangers entire demographics who often wield little power to begin with.
I’d argue that nationalism itself isn’t inherently bad though. Many anti-colonial movements are nationalist, and AES countries have nationalist traits.
I believe Stalin’s “Marxism and the National Question” wrestles with this but I am yet to read it.
You make a good point. Nationalism in socialist projects still deserves scrutiny.