I was referring more to the hardware limitations. The Pi has only 8 digital I/O pins, and I think one serial port and an I2C port? Microcontrollers like the ATMega 2560 (and Arduino Mega board) have 50 some I/O lines (with some analog ones in there) and four serial ports. The 8 pins on a Pi you can eat up pretty quick on a project. Of course you can use the I2C port to add peripheral chips to get more I/O, but microcontrollers in general do this a lot better.