I totally get that. That's why I'm planning on making the service (or at least parts of the service) open source. You can already get insights in the storage usage of the server through status.lngzl.nl (click scan.nsfw.rest). If you upload files, the storage size does not increase.

Also, one of the things I should make a little bit more clear is that it's best paired with data that will be made public anyways (for example, a social media platform) and shouldn't be run on private images (even though it's not storing anything it's probably not needed for those use cases).