Cellulose in an environment with drastic humidity swings shouldnt cause any problems. I paid for access to a study done in Canada that determined that Cellulose was safe to use in unvented flat roof ceilings. (I would post it here, but it is copyrighted) Here is a link if you want to buy it:
http://jen.sagepub.com/cgi/content/abstract/28/3/269
They tested it with a vapor barrier, without, and with no insulation.
Water accumulation over a year in the attic was best with a vapor barrier below the insulation, but without, it made no difference in mold growth. (remember this is the North where it is cold more than it is hot) Borates in the cellulose prevent damage. The water dries out completely over the summer and it was very little moisture to begin with.
In the more southern regions in the states, the vapor barrier wouldn't be good because it would trap water the wrong way during half the year so its kinda pointless. In the deep south, you can put in on the outside of the insulation.
I used cellulose in a flat roof with no venting and no vapor barrier, but it is half hot and half cold during the year.
My insulation contractor told me about a company that installed it wet on the walls, which is normal, and the drywallers did not wait for it to dry. The insulation didnt work at all wet cause the water transferred the heat right through it, but after three years the wet insulation did not mold. They had to tear out the drywall, let the insulation dry and it was OK.
The good cellulose will have glue in it. The glue is water based, so the moisture at worst would activate it and keep it shaped right. (in a perfect world)
Mold can grow on fiberglass, mice will make nests in it. If it gets wet, it will collapse. Bugs can crawl through it all day.
Cellulose is resistant to all this, and you can pay extra to get it with extra bug pest resistance.
Did I mention , I hate fiberglass. YUCK, and yes the new stuff I had to use to patch little spots the cellulose wouldn't fit was itchy. I certainly dont want to breath microscopic glass fibers either.
. what a deal for $250. i would stay away from the glass today. they will be back in a couple of weeks to redo the house which has blown fiberglass that has settled after 34 years to nothing. the cellulose won't do that.