There are definitely mental issues as well, but they change from player to player and manifest in different ways. It's pretty ironic though Siddons' elected skipper is one guy who could do with some confidence (and a technique).
We could discuss high v low backlifts all day, but I'm glad Siddons is at least trying to make a change rather than letting his players repeat the same technical mistakes over and over. This is a change from last season though, when he was trying to change Boam from being an orthodox and technically correct batsman to a power player. The "overcoached" word was bandied around a bit.
Maybe it's not surprising an Australian coach prefers higher backlifts. It would be easier to play horizontal bat shots if your bat is already halfway there. New Zealand has traditionally been all about the front foot though, and the lower the bat is the easier it is to time the ball and play late to fuller bowling with late movement.