At its best, nationalism in some forms can help unify people from disparate backgrounds into an imagined national identity. Unfortunately, most nationalism rots into division instead. Christian nationalism in the US, Hindu nationalism in India, Nazis in Germany. These all reject members of the physical nation in favor of their crazed fantasy version.

Anti-colonial nationalism is fine actually. Not ideal but it is basically "everyone here regardless of more local culture should unite in a combined struggle to overthrow the oppressors" as opposed to colonial nationalism.