I'm fairly sure that it means that the Uno has issues with the code.
SoftwareSerial wasn't used in the Mega version as it has extra serial ports built in and everything seemed to work just fine.
The Arduino Mega has three additional serial ports: Serial1 on pins 19 (RX) and 18 (TX), Serial2 on pins 17 (RX) and 16 (TX), Serial3 on pins 15 (RX) and 14 (TX).