There are other competing free apps, but they aren't great. There's also Sleep as Android, which is a very good app; but it's trialware, and it costs US$4 to remove the restrictions after the trial ends.
Various individuals, such as this one and this one, complain of problems with SleepBot's alarm functionality when using Android 6.x "Marshmallow". They tend to complain about problems regarding SleepBot's smart alarms never ringing at all, or about problems with the alarms' snooze function.
I don't own any Android 6.x device. But I wonder:
Does Android 6.x "Marshmallow" truly cause problems with SleepBot?
Are any of these problems due to "Doze" mode?
Is there any workaround for any of these problems?
Possibly related: "Alarm clocks aren't working on Android 6".