Comment by hakunin

Not the commenter, but the reason is the same as the choice to use direct db driver vs ORM - you get to take full advantage of special features and intricacies of one system, rather than trying to maintain a lowest common denominator of functionality between multiple ones.

You can always build multiple clients however. It could be one thorough integration with one API, and a simpler backup via RubyLLM.

Unless of course RubyLLM supports all of the above, but even then, are they going to be able to keep up as much as a native client in the long term?

The answer is almost always that a native client for a more critical integration is the right call. You can always add another if you need it.