
I agree that Arduino is not a good start if you intend to become a professional. Arduino works really well for the non-technical person who will never progress beyond the arduino ecosystem, and for the experienced embedded programmer who is well aware of its limitations. Beyond that, if your intent is to learn embedded systems professionally, pick up an ST Discovery development system (about $20 IIRC) and have at it. Although I worked for a company that standardized its embedded development on Nordic processors, I don't recommend them unless you need wireless in your project.