Lua is beloved and I have a ton of respect for that project in itself, it's definitely one of the seven wonders of the programming world. But the (well-considered) compromises for its intended use as an embedded scripting language make it pretty rough for large complex projects.
Fennel is just an extremely focused aid to some of the worst lua warts. Pattern matching alone is incredible for the overloaded tables lua uses. The macro system is excellent, and since a lot of the time what you're doing with lua is defining a DSL anyway it gives you more powerful tools for that.
I initially recoiled but I now think it was a genius decision to do it with just a few special forms over the lua semantics. Other than the lisp syntax there's very little new to learn to use it. And the best part is that it hooks into the lua module loader system so you can freely mix tables and functions between the two, a life changer in legacy codebases.
Can't say enough good things about fennel. There are a lot of languages that I like but it's one of the only ones I think is actually good. I rarely write normal lua anymore it's just so flexible.