Not arguing with your assessment of Uber's requirements; but in general, why do you view InnoDB as not much more than a K/V store? And why do you equate clustered indexes with K/V stores?
InnoDB is a complex piece of software, supporting transactions, row-level locking, MVCC, schemas, secondary indexes, crash recovery, hot copy/backup, complex caching and buffering, many tunables, and extensive metrics visibility. Just because it's more appropriate for Uber's rather unusual EAV-like use-case, this doesn't mean InnoDB is a glorified K/V store.
Re: clustered indexes, it's a storage engine architecture choice with well-known trade-offs, both positive and negative. SQL Server also uses clustered indexes and is widely respected among database experts.
Regarding the topic overall, there are use-cases where Postgres is the best choice, and there are use-cases where it isn't. That doesn't inherently mean that other databases are uniformly worse. People like to trash MySQL, sometimes for completely valid reasons, but other times for FUD. But fwiw, several of the major features in Postgres 10 have already been supported in MySQL/InnoDB for a long time, in some cases for over a decade. Of course, that goes both ways; there are awesome major features that Postgres has had for a decade that MySQL still lacks.