In his Wall Street Journal piece on this, Christopher DeMuth (Hudson Institute Distinguished Fellow) had this:

the nationalist claim is…government has abdicated basic responsibilities…:

failed to secure national borders and provide regular procedures for immigration and assimilation.

delegated lawmaking to foreign and international bodies, and domestic bureaucracies…

…acquiesced in, or actively promoted, the splintering of the nation into contending racial, religious and other groups and has favored some at the expense of others.

On the contrary, these aren’t abdications. The first two are by Progressive-Democrat design. The third is a deliberately wielded tool of the Progressive-Democrats to achieve the first two.

One has only to hear the bigoted spew of Ocasio-Cortez, Tlaib, Pressley, Omar to understand this. One has only to see the Progressive-Democratic Party Presidential candidates, almost to a man, favor decriminalizing illegal entry into our nation, and then giving illegal aliens free health care at our expense. One has only to see the Progressive-Democrat Congressmen do their best to block funding for supporting our borders.

The American Heritage Dictionary defines nationalism as

The belief that nations will benefit from acting independently rather than collectively, emphasizing national rather than international goals.
The belief that a particular cultural or ethnic group constitutes a distinct people deserving of political self-determination.

There are no downsides to nationalism.