Have anyone experience with the atom-gdb-debugger ???
some guys from http://1bitsy.org (see also their kickstarter campaign)
make a small stm32 board and developed a plugin for PlatformIO
and also a debugger extension for atom https://atom.io/packages/atom-gdb-debugger.
Maybe that can be integrated in PlatformIO for mbed boards with cmsis-dap and st-link feature?
That is the big missing feature in pio.