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.
The drawback was that the Lunex spacecraft would be significantly heavier than Apollo, requiring more fuel to land it on the Moon and return it to lunar orbit, necessitating a larger rocket to transport it to the Moon.