I don’t know anything about that particular app, but it seems to me that the low user count is just because you can do pretty much the same thing with any of several free options on an android phone.
If you use a custom ringtone, that gives you one set of options. But there are also a number of ways to make a particular song start playing so you just use a Recording of the siren sound.
For example, you can use the android device IFTTT channel:
And here’s a discussion thread where people have talked about various different ways of accomplishing the same thing:
So if what concerned you is the low number of users, my guess is just that there are too many competing methods for doing a minimal alert, most of which are free.
But again I don’t know anything about the specific app.
