Arcadia's biggest drawback was that it was dependent on Unity, which meant that it had all of the issues that Unity had plus some new ones. Without source code access it's difficult to exceed the features/performance of the base engine.
Maybe they've fixed some stuff (or ported it it Godot) since I last checked it, but the general lack of editor support, the clash between how Unity wanted to operate and how Clojure wanted to work, and usual Unity problems kept me from building anything significant in it.
AFAIK Janet is not "much simpler to interface with C++". Janet has good FFI for C, but not for C++ directly. Janet's native FFI system is designed specifically for C libraries. C++ has name mangling, classes, exceptions, and other features that Janet's FFI cannot directly handle. The process would be similar to using C++ libraries from any C-based language, which is far from straightforward.
Jank, if my understanding is correct, in comparison has direct C++ interop. Jank treats C++ as a first-class compilation target rather than a foreign interface.
Maybe they've fixed some stuff (or ported it it Godot) since I last checked it, but the general lack of editor support, the clash between how Unity wanted to operate and how Clojure wanted to work, and usual Unity problems kept me from building anything significant in it.