Zim’s main drawback in these days is that it doesn’t have its own mobile app, and since it’s not based on MarkDown, you can’t use any existing MarkDown mobile apps with it.
That's true, I think they started looking at creating one but it never got off the ground. From my personal perspective I don't, and can't (I've tried), take decent notes on mobile. On the move I just use a jotter it's quicker and move flexible.