Lua itself is great, it runs circles around Python in speed even without LuaJIT, and especially the startup time is almost nonexistent, which is important for scripting. Lisp adds convenience like having everything be an expression and the ability to stick an (if) anywhere for a ternary. And since Lua is so snappy, Fennel has no noticeable overhead. Iirc the Lisp code is compiled to Lua, so long-term there's practically no overhead at all.
Although it's been a while since I've used Fennel, so don't remember if it has any nasty limitations.
There's also a similar thing for Python, called Hy.