The State of the TclQuadcode compiler (2017) [pdf]
tcl.tk
tcl.tk
And don't forget "expect" which is based on TCL also.
Downsides: Oh god TCL is horrible.
Think of it as a C library with a very flexible configuration file... TCL is amazing and was doing what Lua and Node do a decade or more before they badly reinvented a subset of it and called it new.