Every member of my "fleet" is custom - built by me. That means that there are a lot of custom parts. Many times, I use parts I find on THINGIVERSE (3D printer files). But most of the time, the parts I find there are not exactly right for my use. They don't have a hole in the right place, or they need a piece cut off.
Rather than go in an re-engineer the part and generate the proper STL file, I simply drill a hole, or file this, or cut that. The end result is that nearly every part is one of a kind, and even if I can print a new one, it requires modification before I can use it.
Recently, I built a little quad, a little '250 class' fun flyer. It didn't have any fpv or gimbal, but it flew nicely. And it handled well - it was easy to fly. And it was very fast. I was with a guy that I work with (and a personal friend) and I decided to go to a park and show him how the craft flew.
After 'throwing it around' in the air for awhile, I asked my friend if he wanted to try. He took the controls and got it into the air. About a 30 seconds later it was obviously out of control. This was not a control failure but a controller failure (my friend being the controller).
Before I could grab the transmitter, the quad was into a tree and fell to the ground. It broke 3 arms, one propeller, the main body, bent a motor shaft and fried one ESC (I suppose because the motor couldn't turn).
If I had a 'factory bought' unit, I could have ordered new parts and put it together quickly. But since I didn't -. I had to print new parts, modify most of them a bit, cut down a propeller, and modify an ESC (with shorter than normal wires). That took a long time.
There are two morals to this story:
1. Friends don't let friends fly their toys.
2. In many cases, a factory-bought unit is better - especially if you crash one.