The design of all front loaders lends itself to trapped moisture, which is aggravated by the tight seal; the drain engineering and and dark interior (as opposed to top loaders) create a good environment for mold. Warm weather, use of too much or improper detergent and leaving a load in too long after the cycle will increase the chances that you'll get mold/mildew.
Don't worry if it happens, it's easy to fix.
The very simple solution is to order a bottle of Smelly Washer Cleaner. I have no connection to this company other than that I bought their product and it works. They also make a product for towel odor problems (from leaving them in the washer too long) and it works as well.
I bought a bottle about 2 years ago ($14.00 ?) and still have plenty left. Leave the door open on the washer so that the inside can dry out, use a little SWC, and you'll be fine.
It's worth noting that the front loader gets more speed on the spin cycle (horizontal shaft, as opposed to vertical) for better wringing and therefore less drying; it uses very little water and if you follow the directions you spend very little on detergent.
Knowing what I know, I'd buy another, and can vouch for the Whirlpool Duet.
My duet has a trap to keep the pump from ingesting materials. You need to clean that every once in a while. If it gets jammed up with junk and the tub is full there isn't really a good way to drain that tub.