Joins are no fun, yes, but as you gritted your teeth and implemented those cute little table-based key-value stores, did you find yourself mentally calculating the time required to restore the whole system from backup while muttering tiny prayers? Probably not. Did your code wake up the ops team an average of once per month for several years? Did you lose data? Did you have to put up an apologetic blog post? Did anyone have to get on the phone and rescue customer accounts, one at a time, with profuse apologies and gifts? (Now that is a non-scalable process...)

But at least this argument about maintenance is a real argument. The one about wanting to save time during initial development by skipping the declaration of schemas reads like the punchline of a Dilbert cartoon that you'd find taped to the wall in the devops lunchroom.

Slow interconnect between servers caused me headaches in the past with mysql for replication. Shared switched did the same. Problems with locks under high contention did the same. Problems with the client libraries the same. In fact all storage systems have similar problems and pain. Some are just more battle tested than others.