CitusDB has a ton of limitations on your schema/table structure right now. Over time it has gotten better, so hopefully in the future it will be a true drop-in scale out solution. Specifically some types of relationships + indexes are not supported.

True, but as you said, the compatibility is quickly increasing. They actually did a great job and there is fairly little that is not compatible, usually on very advanced side (which is exactly what we would love to have), but the team is especially good at what they are doing and believe they will get there sooner than later.

Oh yeah totally; it's a slow march forward. MySQL Galera took some time before it was ready for serious use, but the fact that it's semi-usable is awesome.

Citus only does sharding, not replication. Either way, this is a fundamental design limitation with traditional single-master relational systems.

Yea, that's just normal Postgres replication, nothing to do with Citus.

Citus is doing sharding and replication: Citus will shard your data and place every shard on e.g. 3 servers. So one server dying is no problem because the data is replicated 2 times.

Ah I see that now. However that's not by default and it still only applies to "distributed" tables, not all the data in the database. Combined with a single-master, it's nowhere near the replica functionality provided by natively distributed storage systems.

Do they have ACID transactions around writing to all 3 servers? Doesn't that cause bad performance?

You might also end up with a LOT of servers unless you starting sharing servers for different shards (but still with different sets of 3 servers for every shard).