[samsk]

Yes, it always depends on application and purpose.

But once you will have to deal with a real FTS load, as you say, you have to use separate instances and replication, use materialized views etc.. and you find your self almost halfway to implementing ETL pipeline and because of replicas, with more complicated setup than having a FTS tool. And than somebody finds out what vector search is, and ask you if there is an PG extension for it (yes it is).

So IMHO with FTS in database, you'll probably have to deal with the almost same problems as with external FTS (materialized views, triggers, reindexing, replication, migrations) but without all its features, and with constrains of ACID database (locks, transactions, writes)...

Btw. I've SOLR right behind the OpenResty, so no hops. With database there would be one more hop and bunch of SQL queries, because it doesn't speaks HTTP (although I'm sure there is an PG extension for that ;-)