I usually don’t interact with Discourse via iOS, but gave it a go and for the most part, it is working as expected. 
To me this feels usable although, admittedly, I’m probably not familiar with all the ways it is/was not working correctly.
The only issue I found was the one already reported here which happens when switching to another app and when returning to Discourse — then the cursor is one line below where one is actually typing. Once that happens, it’s a little finicky to get back to normal state.