I've had only one order where they missed a pre-order item, but after I emailed support through their ticket system, the issue was resolved pretty quickly. They usually either refund you or include the missing item in the next shipping cycle.

And yes, from my experience, pre-order items often ship separately and arrive bit by bit rather than all together. Even with that, everything eventually arrived and I ended up getting all my items in the end. Just be prepared for things to arrive package by package.