The down side was that the Lunex spacecraft would be much heavier than Apollo to carry the extra fuel required to land the entire spacecraft on the Moon and return it to lunar orbit, and consequently a larger rocket would be required to send it to the Moon.
A significant disadvantage of the Lunex spacecraft was its increased weight compared to Apollo, which necessitated a greater amount of fuel for both landing on the Moon and returning to lunar orbit. Consequently, a bigger rocket would be needed to launch it into space.