We love the monorail and have never had any problems with either crowds or getting stuck.
We have had several other "stuck" issues, however.
When I was in 6th grade, my family took a vacation to
Disneyland. Pre-Fastpass, we waited for almost 2 hours to ride Space Mountain and literally when we were finally at the very front, about 2 parties to go, the ride shut down for the day. It majorly sucked (our only day at the park), but it was also kind of cool because they turned the lights on and we were able to go in and see the ride in the light.
A few years later I went to Disney World with my adult step sister and a friend of hers, and I was in the elevator coming back up from breakfast. I was alone in the elevator and it stopped between floors. I was majorly freaked out. It was about 3 hours before they finally got me out, and my "lovely" step sister had gone to the park without me. This was way before cell phones, and she had left a note for me on the hotel room door. I was stuck by myself at our resort all day. Not my happiest Disney memory.
In 2005 at Disneyland, I was on Thunder Mountain one night with my 4 year old DD when it shut down. The ride stopped while we were on the incline going through the mountain at the earthquake. The lights came on and we sat for awhile before a CM came and got us off the ride. My daughter was so excited to be able to walk through part of the ride and see the behind the scenes parts.
In 2007 we were riding Dinosaur for the first time and it shut down at one point in the dark. The car stopped, but we didn't realize it wasn't just part of the ride because the sound didn't cut off and the lights didn't come on. We had been sitting there for several minutes before we realized something was wrong and we weren't supposed to be stopped. For a long time, we just kept expecting the carnotaurus to show up and start chasing us, as it was shortly after we passed him. DD was 7 at the time and was FREAKED out! After about 10 minutes of sitting in the dark with the ride sounds going, we started back up again. It was weird that there wasn't an announcement and that the lights and audio of the ride continued.
None of those situations were what we would have planned and none of them were ideal. I would hate being stuck on the monorail for 3 hours. That being said, it certainly wouldn't keep me from riding it.