As this seems to be a loaded and open-ended question, please don't flame me if I haven't answered it to your liking.
Pi is:
a family of complete embedded systems, processor, audio, video, networking etc.
ARM CPU, (64-bit or 32-bit) quad core.
Targeted to applications and use as a finished product.
Arduino is:
a family of microcontrollers, AVR (8-bit) and SAM(32-bit ARM).
a microcontroller integrated developer's environment,.
Targeted to electronics product developers.
The only things I would develop on a Pi are software applications that run on an OS.
On an Arduino, I have developed many electronic products. Think of the controller inside your microwave, toaster oven, alarm clock, television, soldering iron, car stereo, car dash, car lights etc.