The Arduino software stack is not huge, there is no operating system involved. Our application is the only thing that runs on bare slow hardware with very limited memory. But this also makes debugging harder. The IDE is limited, you debug over serial output. You have to reflash the flash-memory after every re-compile, which can take a minute.
Building a IoT system for very specific tasks that has to run reliable for years without interruption, I would still use a 8-bit tiny ATMega CPU (e.g. Arduino), and to control this tiny CPU and do some networking stuff with a control center using a 32bit ARM CPU (e.g. RPi).
uh, you know that AVRs have debugWIRE (smaller parts) or JTAG (bigger parts)?