You question is very vague, what with talk of sensor networks, battery power, etc, but:
...do you think the Raspberry Pi, Arduino and other open source chips can be scalable for big projects and do you think the technology is reliable?
Firstly, neither the Raspberry Pi or the Arduino employ any Open Source chips. Neither does anything else unless you want to count the RISC V chips from SciFive.
The technology is reliable. The Arduino is after all just an ATMEGA MCU which are used by the billion in all kind of industrial and other devices.
What do you mean by "scalable" and "big projects"? The answer here depends on your intended application.
When it comes to battery powered devices that will run for years, forget it. The Pi is certainly not designed for that. There are many microcontrollers out there that are. See STM32, Cypress, ESP32, etc, etc, etc...
Memory in C++ is a leaky abstraction .