I am going to agree with LoyceV that there are too many potential issues to consider using it.

Most notably the fact that it is a black box for the user what happens under the hood. Too much trust required. For me to ever consider something like this, it would have to be open source.

If I wanted to share private information, I would likely use OnionShare which is open source and rather easy to use.

Your service does seem simple to use. The requirement to download an app is slightly annoying. Having the secret expire after 3 incorrect attempts seems like enough for sharing secrets that are not super sensitive. Expiring the secret after it has been viewed is another good thing. Setting asides problems that I have with it personally, someone might find it useful although I would still warn anyone to trust their secrets with a third-party.