How so? In my mind templates and JSX are equally complex from a user standpoint. In fact I'd argue that JSX is simpler because you don't have to invent slots -- it's an obvious side effect of using JavaScript to generate your structure.
Just reading about slot and slot-scope makes me think Vue's reputation for simplicity is overblown.
It may seem "obvious" to you after being in the water with JSX for a while, but the free-style abstractions afforded by being 'just' JavaScript, like the one you mention, look incredibly complex for someone approaching an unknown codebase.
Just reading about slot and slot-scope makes me think Vue's reputation for simplicity is overblown.