Native web-components
Because Lego is actual native web-components, all its native possibilities (like slots), :host and whatever exists or will exist are on board.
Browser compatibility
Lego is based on native customElements. Support for customElement is spreading and shall increase in time.
When building a web-app you may have control of the browsers. If you’re building a more general website you may need to increase the overall browser compatibility and install the custom-element polyfill.
Dependencies
It is still fully compatible with native custom elements. No magic behind the scene, no complexity, just a couple of useful methods to write native web-components easier.
Using a compiled component has no dependency, nothing extra injected in the browser.