Yes, please do look at them, because Google, Facebook, Twitter, Yahoo, and eBay use MySQL pretty heavily for many of their core storage needs (look at the contributor list for WebScaleSQL). Spotify uses Postgres for the same. All of them also use other things as well, but only for cases where they are willing to make performance or reliability trade-offs like for colder or analytical data. Of course they also use things like Sherpa, Cassandra, HBase, etc... but they lose consistent low latency or consistency or availability when they do so.
The point is, if you are going to bet your business on a technology, it helps if it has been tested with production workloads in many different conditions and scales. You want to know about as many shortcomings as you can. For many of the use cases that people use things like Cassandra for, they will be tolerant of 30ms++ reads and potential read inversions. Redis is used pretty heavily, but it is relatively simple code and you can trace through the entire writepath pretty easily and get a sense of its limitations (being single threaded is a blessing and a curse, you really need to be careful about bad tenants because a single slow query will cause an availability event for everyone). HBase is used in a few places, but usually only for cold data after they expect it to be read only occasionally and they don't want to use up space on their MySQL pci flash devices for it anymore. There are a bunch more, but they all have some latency, consistency, or availability downsides compared to a traditional sharded B+ tree backed transactional store.