The above is good as a rule of thumb indeed.

Another one that I'd add is: - "Are the records in each table in the hundred of millions? Then most probably you'll do fine with an RDBMS".

If you go above that, or you have operations that will extrapolate that number in the billions then you can offload them into whatever non-RDBMS storage you want and do your thing. But that's the thing with RDBMS, you can always move(or offload part of) your data to a non-RDBMS solution afterwards.

But doing the inverse? I wouldn't want to be in that person's shoes ;)