Despite being 6 years in development:
- they basically delivered none of the original promises
- they are extremely cumbersome to use without a library/framework on top (Polymer is the jQuery of WebComponents)
- they are plagued by multiple problems and shifts in the ecosystem (browsers don't want to support `is=`, HTML imports are being dropped in favour of ES6 imports)
- they have multiple problems integrating with existing frameworks: https://custom-elements-everywhere.com (basically any framework that bastardises HTML syntax is ok, anything relying on Javascript only is in trouble)
Don't hold your breath for the next couple of years as everyone tries to figure out how to work with them.