The same old processes vs. threads debate, plus having the ability to scale the coordinator past a single machine. So, if you're OLTP, definitely consider PgDog. OLAP - Citus still wins because of its advanced query engine. We'll get there.
Since Citus v11 (released 4 years ago), any worker node can also work as a "query router" (a node that you can query against [1], and works from this perspective as a pure coordinator:
> for very demanding applications, you now have the option to load balance distributed queries across the workers
You can also setup such query routers as dedicated nodes by setting the `shouldhaveshards` to `false`, becoming an effective coordinator (for querying; not for metadata operations).
So with Citus you can absolutely have as many query routers (coordinators if you wish) as you want.
[1]: https://www.citusdata.com/updates/v11-0/#metadata-sync
Edit: formatting, typo
TLDR: Tokio concurrency > Process concurrency in OLTP.