As I know as couple of people who either build aircraft for a living or build aircraft for their personal use, neither of them is an aerospace engineer. But I'd happily fly in their aircraft.

Irrespective of the title, can they design and build the object in question. This applies to all fields.

The point that we must look at is whether or not the problem before us can be solved by us. It doesn't matter if you build from the ground up or use some pre-built parts. Is the solution going to work and solve the problem at hand. If it does then you have success, if it doesn't then you have failure.

It is a matter of understanding the problem (in terms of the problem space), what solutions you use only matters if you can't solve the problem.