That's great advice! Start out with placeholder assets and get a feel for the gameplay and validate your concept. It's also easier to get good performance by starting out simple and adding instead of starting out big and then trying to cut down to get a playable frame rate. I sort of did this with my bowling game, first creating a prototype in Unity with just primitives (sphere for the balll, capsules for the pins, a plane for the floor), random stock sfx (coin sound for pin collisions), and only enough logic to bowl a frame over and over, but it was enough to try out the controls I had in mind. But something I didn't anticipate is the bare bones game got more downloads than the full game I made later.