I relied very heavily on Day One until they dropped most of their local-hosting features. It was painful and I still haven't found any comparable alternative.
So I literally just moved back to pen and paper. Something I had been doing since school (school school). I haven’t missed any journaling app since. But having one would be nice.
It's not a journaling app per se, but you could use it for that.
The mobile apps are not perfect yet though.
At some point an iOS update is going fully break the app, but I'm still hanging on.