We have to hand it to the team at Motion on this one, they launched an API, and then almost immediately spun up a Zapier integration to make their API even more accessible to others, something that newer tools in the space haven't invested in which makes Motion stand out, and something we greatly appreciate.
The API is surely to improve with time, adding in more functionality, although it does allow for the major things you'd want to use it for already, like create/find/update of tasks, and most importantly, creating the magic auto-scheduled task.
When comparing Motion's API with some of the leaders in project management like Asana, Motion does about 85% of what it needs to do for most integrations. We also know it'll get closer to 90–95% once they allow for custom fields, as functionality around "search tasks by custom field value" is surely to come with it.
The one thing to note though is that the API does not currently work with the appointment scheduling feature. For example, it does not allow for the same API functionality that a standalone scheduler (see in our Cal.com review), will give you on that front. That said, you can build some of this functionality by getting creative using a tool like Mailparser alongside Zapier (e.g. logging new appointments into your CRM upon booking), although I will say, it's not all that straightforward. 😅
All-in-all, Motion's API is getting there, and is likely to do what you'd need it to do on the project management front. We'd only say be aware if you're planning on building robust integrations based on their meeting scheduler, because if so, you might be better off using one of the best scheduler apps in that case instead.