If you can trust the app/platform developers to send encrypted anonymised personal data, then how can you trust them to properly use FHE personal data?

> "we literally cannot read your period data."

If the purpose is aggregated data for statistic, then surely the only per-user data they need centrally can already be aggregated (to some degree) on the device, e.g. send back only statistical-distribution variables of the personal data, for distributions over the 3-4 months? And at some point, does the service need to keep collecting data, once the model is good enough (at predicting ovulation etc)?

Another concern would be: If they are building a model, using user data, why should they own the model and thus monetise it (i.e. sell it back to its users) when users get no compensation for supplying that data in the first place.

A flow-tracking app should just stick to that, and purchase the model (for a fee) from a third party. The third party should concern itself with how to get the data without being able to leverage its position as a flow-app maintainer to trick or mislead the majority of its users into giving them free data.