Calling a model provider's SDK directly from wherever you need it is the fastest way to ship a first version — and it's a reasonable choice for a prototype. The problem shows up once that pattern spreads across a real codebase: the provider's API is now called directly in dozens or hundreds of places, each one coupled to that specific SDK's shape.
Then the provider deprecates a parameter, changes a default behavior, or has an extended outage. Every one of those call sites breaks at the same time, and there's no single place to patch the fix — it has to be found and corrected everywhere, under time pressure, while the product is visibly down.
This isn't hypothetical risk. Model providers iterate fast, and deprecations happen on their timeline, not yours. The question isn't whether an API will change under you — it's whether your architecture absorbs that change in one place or in two hundred.