I have setup the NightwatchJS project with wikipedia sample app. I tried to run that on appium + android adv and it works fine. Then I tried to create selenium grid with hub and node setup and used following config,

# appium-android.yml
server:
  port: 4723
  use-drivers:
    - uiautomator2
  default-capabilities:
    appium:wdaLocalPort: 8100
    appium:mjpegServerPort: 9100
    appium:mjpegScreenshotUrl: "http://localhost:9100"

# node.toml
[server]
port = 5555

[node]
detect-drivers = false

[relay]
url = "http://localhost:4723"
status-endpoint = "/status"
protocol-version = "HTTP/1.1"
configs = [
    "1", "{"platformName": "Android", "appium:platformVersion": "11.0", "appium:deviceName": "nightwatch-android-11", "appium:automationName": "uiautomator2"}"
]

ref: https://appium.io/docs/en/2.0/guides/grid/

After that I changed the config as mention below and was trying to use --env app.android.emulator but each time it gets stuck on Connecting to localhost on port 4444...

module.exports = {
 
...

  test_settings: {
    default: {
      disable_error_log: false,
      launch_url: 'http://localhost',

      screenshots: {
        enabled: false,
        path: 'screens',
        on_failure: true
      },

      desiredCapabilities: {
        browserName: 'chrome'
      },
      
      webdriver: {
        start_process: true,
        server_path: ''
      },
      
    },
    
    app: {
      selenium: {
        start_process: true,
        use_appium: true,
        host: 'http://localhost',
        port: 4444,
        server_path: '',
        cli_args: [],
        default_path_prefix: ''
      },
      webdriver: {
        timeout_options: {
          timeout: 150000,
          retry_attempts: 3
        },
        keep_alive: false,
        start_process: false
      }
    },
    
    'app.android.emulator': {
      extends: 'app',
      'desiredCapabilities': {
        browserName: null,
        platformName: 'android',
        'appium:options': {
          automationName: 'UiAutomator2',
          app: `${__dirname}/nightwatch/sample-apps/wikipedia.apk`,
          appPackage: 'org.wikipedia',
          appActivity: 'org.wikipedia.main.MainActivity',
          appWaitActivity: 'org.wikipedia.onboarding.InitialOnboardingActivity',
          chromedriverExecutable: `${__dirname}/chromedriver-mobile/chromedriver.exe`,
          newCommandTimeout: 0
        }
      }
    },

    selenium_server: {
      selenium: {
        start_process: false,
        host: 'http://localhost',
        port: 4444
      }
    },

    'selenium.desktop.chrome': {
      extends: 'selenium_server',
      desiredCapabilities: {
        browserName: 'chrome',
        chromeOptions : {
          w3c: true
        }
      }
    },

    'selenium.android.chrome': {
      extends: 'selenium_server',
      selenium: {
        start_process: false
      },
      desiredCapabilities: {
        real_mobile: false,
        browserName: 'chrome',
        'goog:chromeOptions': {
          w3c: true,
          args: [],
          androidPackage: 'com.android.chrome',
        },
      },
    
      webdriver: {
        start_process: true,
        server_path: 'chromedriver-mobile/chromedriver.exe',
        cli_args: [
          // --verbose
        ]
      }
    },
  }  
};