Thanks for the blog. We have been using Discourse for two years now as successor to Confluence and it definitely attracts users for the content they find relevant in an easier way. On the technical SEO-side, Discourse works great in general. It performs well, and is easy to have content correctly indexed.
I will share some finding that are still relevant. Most challenges were already known and most have been included in a release. The biggest challenge seems to be better multi-language support, which is probably only relevant for a small part of the installations that target users from multiple countries or countries such as Belgium with three official languages (French, German and Dutch).
Multi-language may seem like a small thing, but only by providing translated content in multiple languages instead of just Dutch the traffic has approximately grown by 400%, significantly better distributed from more countries in our commercial target area.
Keyword Cannibalization
Biggest problem now seems keyword cannibalization when publishing a wiki in multiple languages. Discourse currently out-of-the-box does not provide relationships between topics that contain identical content in different languages. For now, we hope that the ping-back links in posts help Google consider it as the same pages, but we don’t know for sure. More information on Link related topics using link-tag for better helping search engines
Category per language
For multi-language, we now use multiply the real categories with the two languages mostly used (English and Dutch). It works to dedicate a category to a specific language, and Google seems even without hints to make reasonable choices when detecting actual language of a topic, even when the language differs from the meta-data (see Language support on category level for more relevant search results).
BING
Very small issue is the missing language tag for BING (Missing HTML meta-tag "Content-Language" for BING language detection), but Google handles over 90% of search traffic anyway (2018 Search Market Share: Myths vs. Realities of Google, Bing, Amazon, Facebook, DuckDuckGo, & More - SparkToro). I recommend forgetting about it.