I don't build servers or Raid but I'm taking a long shot here as I don't know all the details of where you purchased the card.
But I do know that people will exagerate the truth and even lie to get a sale even the direct manufacturers and resellers of an item.

I'd get another RAID card one that is known to work with the OS you want.
And I wouldn't base my decision on the advice of a sales person unless they had proof.

My opinion, of course.

In my experience if you have driver issues right at the outset even if you correct them you're going to have further issues down the track.