Technical practicality is probably less of a concern. If you build your business on the availability of cheap (or free) APIs, and then these APIs cease to exist and you are left with very expensive alternatives (or worse, no alternatives at all), your business plan may break down pretty quickly.

The same could happen if the API was paid. In this instance, it's conceivable to setup your own NLP/bot engine with OSS alternatives to do more or less the same. But starting with wit would be much faster and allow you to focus on getting the product market fit right