Schema changes need locking... everywhere. Citus is no different. And they need proper design everywhere. If you mean that it requires distributed transactions, well, yes, again: expected and solved. Not even all sharding solutions support this.
> coordinator node
Not sure what the problem is here. If what you mean is that a single coordinator, even with an HA replica, can saturate, that's true, but you can add multiple "query routers" (that's our name in StackGres, see [1]).
> Maybe I am wrong, but I am yet to read stories on operating tens of TB scale workloads on citus.
For example, we have a customer that ingests some 30TB/day, and it's ramping up towards 200TB/day of ingestion. On 24 worker nodes.
[1]: https://stackgres.io/doc/latest/administration/sharded-clust...