Currently I’ve been battling the uncrawlable site. I have prerender.io working, and pages are caching on a Heroku hosted Meteor React app. There’s a sitemap.xml, I have metadata for the usual suspects like description, OG tags, Twitter Cards, there isn’t a robots.txt nor any noindex tags.
It’s on 1.4.4.2, a bit older but seems to be fine in every other regard.