There are some Arduino boards that have microcontrollers only used in one board. While the Arduino Framework, the library of functions, attempts to keep all boards compatible, at the lowest level there are differences. These boards because of their unique microcontrollers will generally have the lowest compatibility and potentially the most problems in their use.
Now there might be some special cases where these would be preferred, but for hobbyists it pays to buy the most universal boards one can. Note also that these are all 3.3 volt boards which may cause difficulties porting from 5 volt (Arduino Uno) projects.
You can find full descriptions of all of these boards at https://www.arduino.cc/en/hardware.
Disclaimer — I’ve purchased none of these boards
Yún Rev 2
This Uno-size board combines an ATmega32U4 with a Atheros AR9331, a MIPS processor that runs Linux. The Linux processor has WiFi and Ethernet connections.
It is considered End of Life, and is currently sold out.
Nano RP2040 Connect
This Nano-size board has a Raspberry Pi RP2040 microcontroller. It is the only Arduino board that relates to the Raspberry Pi, however this is the low end microcontroller Pi. It has WiFi and Bluetooth through a Nina W102 module, and also has an IMU, microphone, and temperature sensor. Probably best to stick with the conventional Raspberry Pi products for best support.
Nano 33 BLE (Sense)
These Nano-size boards have a nRF52840 microcontroller. Arduino intends this for AI Edge Computing and has an IMU, microphone, proximity/gesture sensor, barometric pressure, temperature and humidity sensors. Hey, it uses the AI buzzword. There is also a NINA-B306 for Bluetooth. The non-“Sense” version of this board does not have the sensors.
The microcontroller runs Arm® Mbed™ OS. While this board does look interesting, keep in mind that it is a one-off design.
Due
This is a Mega size board, and the only one with a SAM3X8E ARM® Cortex®-M3 microcontroller. It is certainly one of the fastest Arduino boards and has plenty of digital pins thanks to the Mega form factor. But, again, it is an old, one-off design. Ignoring price, it should probably be considered replaced by the GIGA. However even that is a one-off design, but at least it is new.
GIGA R1 WiFi
This is the latest Mega size Arduino board. It uses a STM32H747XI microcontroller which has two different ARM cores. This is the same microcontroller that is in the professional Portenta H7 board. It also has a Murata radio module for WiFi and Bluetooth.
The microcontroller runs Arm® Mbed™ OS.