I looked at mbed five years ago , was there not a big catch that you needed some special bootloader or something?
Most boards supported by mbed have a debug chip on them that connects to the SWD and UART pins of the target microcontroller and provides drag-and-drop programming (it appears as a storage device), serial interface (USB CDC device) and the debug interface.
Because of this there is no bootloader required.
You can use most of the mbed-enabled boards as a debugger. For instance, the STM32 Nucleo boards allow you to connect to an external target device for programming and debugging.
But the mbed library itself does not require the debug chip, and there is support for several boards that don't have one.
Many people use an mbed-enabled board as a debug interface to their own boards. We've done this on a number of projects.
You could use another debug interface with mbed (I've also used the SEGGER J-Link for this).