Rust, ECS-core, slimmer abstractions, less tooling (less to learn, but also less pre-built), less complex as a general rule since its less featureful, which also means less cruft/bloat; unreal engine also has a general preference towards "realistic graphics & FPS gaming", to whatever detriment this means for any other type of game (e.g. afaik 2D games are not well-supported or tooled, and probably never will be; graphics tooling can be dramatically overcomplicated when you're chasing something stupidly simple, like replicating King's Field style billboard-3D graphics), etc

The big three specifically, which is pretty close to the reasoning for choosing any game engine thats not Unreal/Unity probably would be:

1. Rust

2. ECS

3. More likely to understand the damned thing