I don't see what about front-loading washers would make them especially prone to standing water or mold. I would expect that vertical washers could just as easily have standing water. Worse, it's probably out of sight behind the drum where you can't clean it.
Because the door is below water level, it needs to be sealed. There is a rubber gasket submerged in water that molds. Top-loaders don't need a rubber gasket.
I've owned both, and the brand-new front loader I just got is definitely worse in several areas with regard to collecting water.
The door gasket is pretty bad (not sloped well enough for water to completely drain away from it), but it's also got a pull-out tray for detergent which also collects quite a bit of water each time.
I did have a top-loader that got a bit musty smelling, though that was largely my own fault- living alone as a bachelor at the time (infrequent use) and habitually starting a load late at night only to fall asleep before I took it out.
I'm not entirely sure what it is. But it's actually a common problem with front-loaders. If we're not extremely diligent in hand drying the washer and leaving the door open, our clothes will smell REALLY musty. It's really bad. The internet is full of anecdotes.
I've never had the same problem with top-loaders. My next machine will most definitely be a top-loader.
I can confirm having owned 2 front loads. We leave the washer door open all the time while idle - sometimes blowing a fan inside to force-dry it out. Otherwise, mold city. It doesn't matter which make/model you buy, they all exhibit the same problem.
You still [probably] need to hand-dry the gasket, but according to my research it's what manufacturers recommend when pushed. I order a box on eBay every 1-2 years, approximately.
