NOTE: Since copying assets is now fully supported, this project is now archived.
The default Angular build architect @angular-devkit/build-ng-packagr for ng-packagr doesn't copy assets of libraries.
Most library projects have assets which was mentioned here
/angular/angular-cli/issues/11071.
This Angular Build Architect solves the known issue of @angular-devkit/build-ng-packagr even if you only copy assets.
Install @linnenschmidt/build-ng-packagr into your angular project.
npm install @linnenschmidt/build-ng-packagr --save-devor
yarn add @linnenschmidt/build-ng-packagr --dev- Replace the build architect of your libraries by  @linnenschmidt/build-ng-packagr:build."architect": { "build": { "builder": "@linnenschmidt/build-ng-packagr:build", 
- Add your assets glob rules to the options section like as you normally do for apps
"options": { "project": "projects/lib/ng-package.json", "tsConfig": "projects/lib/tsconfig.lib.json", "assets": [ "src/assets", { "glob": "**/*.css", "input": "src/some-assets", "output": "assets/some-assets" } ] } 
A final angular.json file could look like the following example:
{
  "$schema": "./node_modules/@angular/cli/lib/config/schema.json",
  "version": 1,
  "projects": {
    "lib": {
      "root": "projects/lib",
      "projectType": "library",
      "architect": {
        "build": {
          "builder": "@linnenschmidt/build-ng-packagr:build",
          "options": {
            "project": "projects/lib/ng-package.json",
            "tsConfig": "projects/lib/tsconfig.lib.json",
            "assets": [
              "src/assets",
              {
                "glob": "**/*.css",
                "input": "src/some-assets",
                "output": "assets/some-assets"
              }
            ]
          }
        },
        "test": {
          "builder": "@angular-devkit/build-angular:karma",
          "options": {
            "main": "projects/lib/src/test.ts",
            "tsConfig": "projects/lib/tsconfig.spec.json",
            "karmaConfig": "projects/lib/karma.conf.js"
          }
        },
        "lint": {
          "builder": "@angular-devkit/build-angular:tslint",
          "options": {
            "tsConfig": [
              "projects/lib/tsconfig.lib.json",
              "projects/lib/tsconfig.spec.json"
            ],
            "exclude": ["**/node_modules/**"]
          }
        }
      }
    }
  }
}
Test package
yarn testBuild package
yarn build