It is more difficult because it's a tight celular structure. If it's not sanded properly the stain will be darker at the sanding lines. This results in a swirly finish or what looks like many fine circular scratches.
In this case, if a sub says it's a bad idea it's because he's not confident in his ability to do it. Doesn't make him a bad sub just means if you want to use him you should follow his advice.