I've been using this and a custom React wrapper at work to build an automation tool for legacy Windows apps with a lot of success: https://etacautomation.com

Blockly is vanilla JS and very easy to extend as needed (I added a better type system, for example), and their new plugin system is super helpful. Its performance isn't quite there, but I have high hopes for the future with the changes I've seen so far. The devs have been wonderful to work with, too.