I use rewatch with a ReScript 11 monorepo for some time already and in the context of a monorepo it’s a game changer! A couple of seconds + less than 512MB RAM (haven’t tried to push lower) to watch/build all the ReScript code and then multitude more time + 2GB of RAM to compile the remaining 10% of codebase with tsc.
What I was missing a little is the bulk code code-formatting across the whole code base, but now it’s coming with ReScript 12 
Many thanks to people involved for the hard work. Really-really appreciate this 