The UI is very configurable and has many power user tools, like their search is very good and has so many features.
The Language for writing text is bad and sadly not even the same on other Atlassian products. Please just let me write markdown.
Confluence is another story. It is one of the better tools for documentation, but it's generally slow no matter who runs it. It has very quirky drawbacks like a page needing to have a unique name in a space even if it is in a completely different tree structure.
The editor itself from confluence hangs so much and has destroyed pages multiple times for me, thankfully the history is decent and you can recover usually.
