This is a great idea, and a welcome addition to the space. Fitness data standards are exceptionally poor and there's very little alignment between the major data collectors. This has resulted in the creation of a small army of syncing apps, developed to move data around between the most popular platforms, but as far as I know no ones attempted anything like this.

Having spent nearly a decade trying to extract data from the various walled gardens in the fitness space, I think something like this has really only been possible in the last few years as some of those walls have come down just a bit. That said, there are still some enormous hurdles to overcome. From the documentation it's not clear how OAuth is implemented. The authentication standards for many of these platforms are very diverse and often unfriendly in terms of the API agreements, rate limits, and various OAuth standards.

I'm assuming since it would be a violation of most API terms to pass data to a 3rd party, that the consumer of Terra must first acquire their own API key and then Terra uses that on their behalf? Although, if that's the case, OAuth implementations must be very tricky, especially for APIs like Garmin's which uses OAuth1a.

Nevertheless this can only be good news for the cause of open data. If it's successful, hopefully it'll create some pressure for these companies to expand their capabilities and relax their terms of service.