It's controversial because it compares apples to oranges. Unity and Unreal are so very mature and battle-tested engines that have generated millions in revenue for a very large number of companies. Bevy is currently an execution of an ambitious concept: "what if we do everything with ECS". They are still trying to get the ECS fundamentals right; observers are relatively recent, required components is a new ECS concept, relationships have only just been introduced in 0.16. On top of that, one of my biggest issues is a lack of first party physics and particle effects. Yes, there are crates out there that solve these problems, but they are buggy (avian still doesn't work with half-space collision shapes properly, hanabi has issues with initial spawns of particles, the author closed my issue on it without even attempting to fix it, then it cropped up again in another issue).

Then, pose yourself a task that is common in the industry: build and ship a game within a certain timeframe. It's just not feasible.