I've seen a lot of refusals for people from cultures where wedding ceremonies are complicated and very important. If people leave out some of it (to do later with more family or whatever else), there's a risk officers won't see it as a genuine marriage, for example. Very fast marriages, after one year or less of dating, relationships with a big age difference (I've seen some with 30+ years difference get approved as well, though), lack of a common language both are proficient at, not giving enough or satisfactory proof, etc.
But there's sadly definitely a trend - those from non visa exempt third world countries have a much harder time. It's not that often that someone from the US or Germany gets rejected, especially for a bogus reason, you know? The thread is mostly populated with people from Africa, south Asia, and the Middle East. It seems there's also cultural gaps at play - officers want to see A, but culturally that A never happens so then what, etc.
There was a really infuriating story a few weeks ago where someone from the Middle East got refused because the officer saw on their pictures that the lady was wearing the ring on the wrong hand and that made her think the marriage wasn't genuine. Only all of those pictures were selfies and selfies mirror the image when you take it. It happened in an interview and they take your phone away so the lady wasn't able to show the officer that mirroring effect. She asked if she could bring in her phone, but nope. And so they got refused.
The system works well and as it should for the most part. But like with anything else, there are those who fall through the cracks on both sides - I'm sure there's people who managed to game the system and there's also people in genuine relationships that have to appeal rejections.