Yes, multiwii firmware (2.5) actually needs more development but I dont know why the developers then stopped. Perhaps, it could be caused by the capacity of Arduino board - since I see many gets esp32, STM even raspberry, etc for robotic projects. But, As a beginnner, I try to optimize, modify the firmware the best I can.
I dont know, no esp8266, no esp32, no stmX with my current projects. All is about arduino board (Promini, UNO, Nano, etc - LOL).
IN FACT, with arduino board, my diy drone and another projects runs powerful and easy to do maintenance 
Note: I've been building Racing quadcopter with MULTIWII Firmware now. And later, I and my friend will do drone racing.