All “hub” devices that are meant to unify the functions of a home's “smart devices”, presumably all from the same manufacturer as the hub. These devices appear to mostly be Linux/ARM-based.

To ensure that the bootloader can not see the environmental variables at boot, timing is critical

This little “smart hub” (presumably, a device linking together a series of smart home devices) has a method which affords the user root access. From there, one can also enable SSHing to this device.

https://www.exploitee.rs/index.php/Staples_Connect_Hub

this causes U-Boot to drop into a interactive shell over UART

Many methods exist for this “Smart hub”, although only one appears to still be functional after various firmware patches. It involves shorting the NAND’s pins upon kernel reading. From there, a root shell can be accessed by altering kernel arguments.

https://www.exploitee.rs/index.php/Wink_Hub