There is nothing wrong with simple engineering as long as it's effective. Previous Classics and Universe toys have simple but effective engineering, you can transform them quickly and both modes are satisfying for the most part. Most Movie scouts also had simple but satisfying engineering.
By the sounds of it, it seems that FOC Optimus is not only simplistic but has overall an unsatisfying design making it disappointing.
Simplicity does not make a toy disappointing but ineffective and lazy design does.