With scratch we mean that you don't lineart and bevel it and then extruding some faces. For example, if you lineart + bevel, put over that motor image then it looks like it is 3d, but if you zoom in into a tube then you see that the tube is flat. Know what Im meaning?
The thing is, with your method you see the details but things like tube, wheels, seat etc are just flat when looking into it.
See it just as a cake with candles. You take a picture of a cake from above, meaning that candles and such are also on the picture. Then you are linearting the cake, bevel it so you get a circle. Then you paste over the texture of the cake and then, from above perspective you see that the cake with candles is on and it looks like there are candles on it when you are viewing it from above. But from sideviews, you see that there isn't any candles, just only the circle.
I also do lineart + bevel, but I work in parts. Other people work with face extruding. Just see which way fits you, but still, the reason why people dislikes it a bit is because it's easy. Everyone can lineart it, bevel it, paste texture over it but that doesn't give you really the feeling like it should been, if you know what I'm talking about.