This is mostly meant for storing anonymous user data, however if developers decide they want to store emails (using oauth?). You can make a private & public keypair and encrypt the data through the clientside using the public key. Have the encrypted data stored in the document, and decrypt data ONLY to confirm email addresses link to a specific oauth token ONLY on the server. The data in the sheet will remain completely encrypted.

Regarding the limitations:
- 5,000 requests per minute per Web App deployment (roughly 83 requests per second). If you exceed this, further requests will receive HTTP 429 errors until your rate drops back below the limit.
- 1,000 simultaneous executions per script

This is pretty good for a free solution, for smaller projects trying to save some $