For now, it is theorethically compatible with C64, Atari 800 and Lynx, but works much better on the C64.
As it uses 8bit-Unity behind the scenes, it should be possible to adapt it to the other platforms supported by that tool. Unfortunately, BlocklyVN8bit still hasn't been adapted to work with newer versions of 8nit-Unity; in other words, NES support is theorethically possible, but still not implemented.