Column-oriented stores normally have just as much schema as row-oriented stores. What they're faster at is scans where the full row tuple is not required to compute the result. Storing the data in columnar format rather than row also means much better compression: similar data is brought closer together.
Column stores are great, for some tasks and have disadvantages in others. Redshift was I believe the first implementation at top of PG so it's doable, however not sure that PG itself is a good engine for it.
In our case, where we need to have access to multiple columns at once, column stores (including the ones that clump them as families/groups) proved to be slower and required more resources than the row-oriented stores.
I think this really goes case by case based on your needs. If we would benefit from column-oriented DB we would not chose PG/Citus but something else (probably would stay still on Hbase).
I know that Heap Analytics is using jsonb to have schema-free setup on citus and I think it's working well for them.