Assuming "at scale" means you have no other option than horizontal scaling. At this point differences between databases start to shrink. Most of the complexity

Assuming "at scale" means you have no other option than horizontal scaling. At this point differences between databases start to shrink. Most of the complexity and features are being moved to the application layer, and your database is becoming dumber. More subtle (and frankly boring) features are becoming important. Like how cost-effective is it to run, observe, backup/recover, etc. Honestly, I feel like most interesting stories are before "at scale". Stories about how you delayed the need to "scale" for several years (and probably saved the company by allowing to focus on product instead of infra). But unfortunately those stories aren't something people tend to brag about. Nobody writes stories about how they chose to be pragmatic and not scale because they could just buy a bigger server or add a caching layer.