1) Radiation

2) Partial gravity

Can they go more than 6mo without resupply? I am picturing a moon base that is only resupplied every year or so. I seem to recall that Mars only comes around every 18mo? I may be wrong on that. The ISS is not a ground based structure. So lots could be gleaned from having a base where astronauts enter and exit regularly. Also, ISS gets a different amount of sunlight. So I don't think there is a valid comparison.

To be fair, the vacuum on the moon is also not comparable to the windy atmosphere on Mars.