It may theoretically have a rating of 1 A, but only when bonded to a substantial heatsink. The on-board regulator - unless you use a variant such as the 'RoboRed' (it has a proper switchmode regulator) - has no heatsink of significance. Once you start connecting things that draw current, and that includes connecting things to draw current from the '5V' pin, you are playing with fire - or not! This is even worse for the Mega as it has more pins to connect things to! The 'Vin' pin or in fact the 'barrel jack' which is the same with a diode, was provided to demonstrate the Arduino. Essentially nothing else as the regulator, whatever its rated capacity, has no effective heatsink and will overheat and (hopefully safely) shut down if required to provide more current than the microcontroller itself and a few indicator LEDs at 10 or 20 mA each. This is true - there is a regulator on the board and it can be used under limited circumstances to power the Arduino board and only the Arduino board. The confusion is encouraged by descriptions in the Arduino references that there is an on-board regulator on these boards. This was the official information I've been working from People liked the Due but it never was anywhere near as popular as the AVR boards and then Arduino discontinued it.I understood that an Arduino Uno r3 can accept any voltage from 6-20v and that the recommended input was 12v? The SAMD and ESP8266 boards have been very popular recently and so there are quite a few libraries for them. The Arduino AVR Boards such as your Uno have been around the longest and they are the most popular so you will find that most Arduino libraries are compatible with AVR. There are a significant number of hardware specific libraries that are not compatible with the Due. Some hardware specific libraries have been specifically written to support the SAM architecture. Some libraries don't contain any hardware specific code and so can be used with any board. The Arduino SAM Boards hardware package that you install to add support for the Due to the Arduino IDE comes with SAM specific versions of some of the official Arduino libraries that are hardware specific.

Some shields have IO output levels that are at 3.3 V or less. Some shields shift their levels according to the voltage on the IOREF pin.

Some shields don't output to IO pins at all. It is not compatible with any shield that will expose the Due's IO pins to 5 V.

Arduino due shield multisim