Every time I hear someone make a claim like this I have to wonder how much experience they actually have with these systems in a demanding environment.

Relational databases, all relational databases, have a disturbing tendency to fall over suddenly under load with little advance warning. They don't have a pleasant gradual failure mode. Instead some point of contention goes from 99% of capacity (at which point it takes very little load) to 101% of capacity (at which point everything falls apart).

If you've never experienced this, then I'm pretty confident that you've never scaled a database to its limit.

Which isn't that surprising. Most companies don't have sufficient load to make their database break a sweat. But once you've encountered the region where problems happen, life gets much more difficult.