Gutenberg just needs a little hand.
I can confirm that centering of most media embeds does not work.
Left and right alignment do, but it then causes content to wrap around the media in an undesirable ( for me ) way.
Here is some SCSS that mostly worked for me. There is some spacing on the right that I haven’t tracked down yet so the Tweet is not 100% centered.
This works for Twitter, Instagram, and Tumblr, if there is something else let me know.
Facebook Posts have some funky static widths applied in a few places so I haven’t been able to adjust that one yet.
Facebook Video I treated as a responsive embed (100% width), You can check that out here.
.wp-block-embed.aligncenter.is-type-rich{
> .wp-block-embed__wrapper iframe,
> .wp-block-embed__wrapper twitter-widget {
margin-left: auto !important;
margin-right: auto !important;
}
}