They seem like a huge step up from the arcane "1980s Unix" nightmare of Postgres clustering but I don't hear about them that much. Are they not used much or are their users just happy and quiet?
(These are all "NewSQL" databases.)
We run them on dedicated hosts or on Hetzner cloud instances. We tested out RDS Postgres, but that would've literally tripled our cost for worse performance.
Only had a few hiccups with the big cluster but they were resolved quickly with their support.
We're very happy with the product, and have leaned quite a few optimization tricks to get the best out of it. Easy to use as well, join the nodes and it just works.
It's not perfect though, we've had quite a few issues with deleting lots of data at once, it doesn't like that. So we have to do deleted in smaller chunks.
TiDB is a pretty interesting project, but there are a few limitations that should be taken into account when trying to use it: https://docs.pingcap.com/tidb/stable/mysql-compatibility
A lot of these are tradeoffs that will affect how a database can be architected, such as having no access to foreign keys and thus needing to think about any sort of consistency and not leaving orphaned data at the application level.