Being a huge supporter of TestNG I still see it is treated as being number 2 by tools authors. Many tools and libraries support JUnit from the very beginning, but you have to wait till they also implement support for TestNG. This might be a serious drawback if you plan to use some latest buzzing technology.
Over the years situation improved a lot. For example this used to be an issue with Spring, Gradle, or Maven Surefire but is no longer a problem because they all support TestNG now. Also all IDEs are treating both frameworks equally.
So, make sure the other technologies you plan to use play nicely with TestNG. This is rarely a problem, but still better make sure of it ahead.