In replicated Mongo scenarios higher write volume increases the probability of inconsistent reads. What this means is that there's a chance that on some data—no matter how safely you attempt to write it—you'll end up in a totally inconsistent state for your system.
The actual impact of an inconsistent state is very hard to judge. It could be as minimal as having two different users just see something weird on their screen for a moment and then it goes away. It could even be totally avoided if your application handles inconsistent data well.
At the same time, it could also cause complete and nearly untraceable complete corruption of all data in your system. Who knows?
It'd be a bit like building a bridge using metal with a known defect. It'll probably work fine for a long time and depending on how and where that metal was used you might be alright.
Or you might have a complete structural integrity failure at any moment once stress starts ramping up and you'll just have to blame it wholesale on using bad materials.
