Why not? Plenty of serious companies use DDB or MongoDb or plenty of other databases for their "serious applications"... this seems like a rule based on a bad experience. Would love to hear about it.
Why not? Well, a caveat: I will always use an RDBMS when the data is inherently relational. In a "serious application" (I realise the term serious is hyperbolic here and I shouldn't have used it) this is true most of the time.
If it weren't true, then of course I might consider using DynamoDB or similar. But I think these situations are few and far between.
I'm often wrong though and happy to learn about some counter examples if you have any / have my assumptions corrected.
