If you are using cellular for monitoring, keep in mind that NextAlarm has the panel report every time you arm or disarm the panel. They don't do nightly tests like some do. Assuming you leave and return once a day and use the alarm at night that is at least 4 minutes a day (or 2 hours a month) it will be higher if you leave more often.
The ABN that Next Alarm uses has a heartbeat, and NextAlarm will send you an e-mail if the heartbeat is missing for a period of time that you set, typically 5 minutes. In addition, if the alarm panel attempts to get to NextAlarm but it can't because of Internet problems, it will will report a communications error just like if the panel used a phone line and the phone line was down.
So with Internet monitoring, the connection between the ABN and NextAlarm is supervised, in a sense. If you use a analog phone line or cellular phone connection and that connection goes down, you'll never know it unless it occurs at the instant the panel is attempting to communicate, like when you turn the alarm on or off.
It is also true that some panels can monitor the phone line voltage and report if the line is cut, but a phone line cut and not working are two different things.
So, Internet monitoring is really no worse then monitoring via a analog phone line, and its probably much better.
BUT if you use Internet monitoring, make sure your modem and router and switches all have battery backups, or if the power goes out...