Not saying they are unreliable per se, but making them really highly available is much, much harder than some NoSQL stores designed for HA, and the solutions are much more complex, usually beyond the point of being able to prove their correctness. It will cost you lots of effort, money and hardware. And to make it network-partition-tolerant, you'll have to give up ACID anyway, so one of the main advantage of RDBMSes over NoSQL stores goes away. IMHO not worth the trouble. In fact, most of the RDBMS systems operating in banks and insurance institutions I've seen were not even fully ACID. They were AD + eventually C and very relaxed I. You really don't need full ACID to handle money, it is just a convenient model for programmers.

I can see systems based on RDBMSes, even the most expensive ones, claiming 7 nines availability on paper do actually fail and sometimes in a totally weird ways, that fixing the mess takes too long. I know some of the companies migrated to NoSQL stores exactly because of this reason - an expensive RDBMS cluster failing after hardware accident while another NoSQL cluster still operating fine in the same datacenter, despite networking problems. I've seen that simply way too often happening to big names, including a few commercial banks and telecoms in Poland, to believe in marketing of HA RDBMS store. Sure, all of them recovered (sometimes after minutes and in one case after a week) and none lost any data, therefore I'm not saying RDBMSes are unreliable ;)

This is exactly a similar story as with scalability. Can RDBMSes be scaled? Yes, they can. But it is expensive, hard and requires very careful application design. It does not work automagically by "I'll simply normalize and throw my queries at it".