I've used it on and off. It's ok and I was happy. But it's a fairly thin layer and a leaky abstraction, so I ended up looking at docker logs and firewall rules more times than needed. Unfortunately when things were down, noting the context was not a priority, so I don't have specific issues raised to show.
While in some cases it is a quite thin layer - some commands just proxy along to the underlying Docker command + some flags - I'd love to hear more about places where Dokku itself wasn't useful for debugging your deployed system. Please feel more than free to reach out via Discord/IRC/Slack[1] (all three are synced) with more info if filing an issue is a big reach. I'm more than happy to help folks debug live/file issues as a result, as it makes the project much better.