Relations are great, and the UI makes it really easy to see what’s happening.
Also agree with UI issues mentioned by @gustavpursche - Having them squeezed into the sidebar is a bit difficult, and you cal only see the ID so it’s not always easy to check the related content/record without opening it in a new tab. Some of this will probably be tackled in v4 but definitely would be an improvement to fix this and related content searchable and provide access to more fields, or maybe a “quick view” panel for the content.
Also got ambitious and built a raffle style game site called giveawayking.ng where people can log in and claim various giveaways. Every giveaway has many “claims”, claims also belong to users. But noticed some lag when fetching records. I think this is related to Knee error, but when fetching thousands of user claims for example, the server really struggles, working around this using a claim field and pushing all user IDs to an array as a temporary fix. But it would be nice to use relations for this all the way.