I'm an indie game developer with over 10 years of experience. I'm not using Unity or Unreal, but I do have some frameworks that handle things like rendering graphics or playing audio. However, I write all the game logic, data manipulation, and entity management from scratch for each game I make. It seems to be the smoothest option for me, since I have full control over how my games actually operate, but I don't have to reinvent the wheel as far as the low-level architecture goes.