One positive thing I can say about Weaviate is that it's incredibly simple to get started with it, as it handles talking to the vectorizer(s) for you, and they provide many out of the box container images for them. With that we were able to integrate a similarity search into our product very quickly.

From what I can tell, its competitors generally don't do this, and you have to handle generating the embeddings and all the back and forth with the embeddings yourself, setting the barrier for integration much higher.

If you are at a level where you want to get a lot more control about embeddings, I agree that there it will often be better to just use the vector search capabilities that your existing solution like Redis/Postgres are getting.