We accured lots of downtime due to mongo.
But later versions were rock solid and I've matainer mongo installations at many startup and SMEs once you setup alertd for disk/memory usage, off you go. Works like charm 99% of the times.
Not that that's really a surprise or was unknown, it's just fairly new to see in the open source ecosystem instead of the enterprise one.
SQL has decades of production maturation, and has wider domain knowledge.
Unfortunately the number of my customers who would sign off on just "two nines" is approximately zero...