Back in ~2012 I interviewed at a company that chose Mongo as its database, and my white boarding question was to implement joins in nosql as that was a recent problem they were solving. After doing the problem I asked innocently, “but all your data is relational - why use mongo at all?” And the CTO went red in the face and exploded, actually yelling at me about mongo’s many benefits (just not, ya know, anything a normal database provides). Needless to say I didn’t get the job!