My thought so far...Performance lacking.
I am on a Cyanogenmod 11 Nightly Sprint Galaxy S4. I REALLY wanted to like Themer. It looks AMAZING.
However after using it for a couple days and trying different themes, it came down to one thing - performance.
It just seemed sluggish. When I press the Home button, it took up to 3-5 seconds before switching for the active program back to the Home screen. Then I was frequently seeing a "loading" message.
Secondly, the time it took from tapping an app icon and actually seeing the app was pretty delayed.
My benchmark is that my current launcher is Go Launcher Ex. It's really quick and responsive. Match the performance of that, and I'll be switching.