Well there seems to be a lack of understanding as to what you and many others like you do exactly? From your other post it seems you’re learning the engine from source code, which is great. But the general use case scenario of an engine is to make games… So yes if you want to be an engine developer and learn how AAA makes engines, I’m pretty sure Unity would be a dead loss for you.
But for people making games, Unity can be a great tool. I don’t think Unity is quite as bad as some make it out to be, neither is it great either…