how are mini-series (in and out of continuity) structured? Like, how much control does a writer have in the story of the book as opposed to an ongoing series? Do you believe they are more difficult to write?

No, I think minis are easier to write, they are a complete story. Writing the same characters and keeping someone's attention for years is a very specialized skill and not always very easy. The difficulty with minis is trying to have some real punch, some real impact, in four short issues (or however many it is).  But I love that, I love the economy you have to use with a finite story.

Different writers and different projects vary significantly in how much control is exerted.  If an editor asks me or, say, Scott Snyder, or Matt Fraction or whatever, they know we have an audience and a viewpoint and some experience…there is a bit of trust that we know what we want and how to get it. So we are generally, but not always, given more slack to accomplish what needs to happen.

If you are newer, or your work is a little less singular (and I do not mean that as an insult), then you may have a lot more control exerted. Also, tie-ins tend to offer the writer less individual control.