Phlox wasn't immune, the process just moved more slowly through his body. I also remember him refusing to eat.
Then he exposed himself to lethal doses of radiation.
Regeneration made the Borg scary for the first time since The Best of Both Worlds I. That makes it a winner in my book.
I hadn't meant immune as incapable of being assimilated, but all species we've seen so far (except 8472) were assimilated within seconds just not Denobulans apparently, its one of those handy plot elements where one person isn't affected by the space anomaly/alien of the week (which really had been done to death in Trek now).
Have to admit I didn't find them scary in the episode, I was lamenting at the need for ENT to dredge the Borg up and into a story rather than doing something interesting.