Verify InReach Mini is working correctly

Anecdotally, TTFF (time to first fix) on the Mini from a cold start is longer than some other iR devices. Similarly, reacquiring after a warm start takes longer. Because the Mini turns off the GPS radio except when it NEEDS a fix, the warm start performance is much more important than on other devices. 

Note well: The Mihi does not use an EPO file. This means that TTFF on a cold start depends on having a valid almanac on board for the current date/time/location. If the unit is off for more than a couple of weeks or if you move it (while off) more than a few hundred miles, the almanac will not be valid. To fix that, turn the unit on with a clear sky view, wait for it to acquire (full sky search will take several minutes), then set the unit down and walk away (so you are not blocking any part of the sky). Let it "soak" for 20 minutes or so to acquire the almanac. After that, you should see much improved TTFF. I suspect that many report of poor acquisition performance are because of this.

The ideal carrying position for the Mini is high on your shoulder (for example, on a pack strap). This minimizes issues with acquiring, as well as facilitating efficient communication with the Iridium satellites.

In my experience, the real power hog is not the GPS radio. It is the iR radio. When sending (track point or message) and when doing an active message check (due to user request or automatically once an hour), the Mini will (re)try VERY hard to make contact with the Iridium satellite. In challenging conditions (or if you do something silly like toss the unit in the bottom of your pack), this results in a lengthy sequence of retries. Because the iR radio is transmitting, it uses a lot more power than the GPS radio. 

In my limited experience, the Mini runtime on a single charge comes very close to Garmin's claims - assuming that I carry it as described above. And assuming that I am not at the bottom of a canyon (urban or otherwise) or in some other challenging situation. YMMV.

My unit has been reliable. When you read forums, Reddit, and so forth, keep in mind that those who have problems are much more vocal than those who do not. This is not to say that problems don't exist - only that they are probably over-represented in public forums.

Assuming that you have activated the unit on a messaging plan, you can send a small number of free test messages each month. You will receive a robo-reply to each such message. You can also send messages to yourself (to an email address or phone that you control). These will be charged against the plan.

Before starting a trip, you should either do a test message, send one to yourself, or send one to one of your contacts. Never hurts to be sure. There are other reasons to send to your contacts before a trip. This lets you know it's working before you start. Not quite the same as whether or not you can "rely on the unit". It does not consider power consumption or the possibility that the unit will die two hours later.

People who are concerned about power consumption typically carry a solar or battery pack charger for use in the field.

Leaving the unit outside, even with a good sky view, is not valid runtime check. The unit will sense that it is motionless. It will increase the sent track point interval to 4 hours (so as not to use the iR radio to send useless track points). It will wake up at the configured send interval to check to see if it has moved, which involves the GPS radio. It will also continue to do the once-an-hour active mail check. The latter is why the battery will run down quickly if you do this inside a building.

The only indicators of problems are (a) failure to acquire a fix in a reasonable amount of time and (b) the status line indicator that shows iR radio activity (which will be "on" for long periods if the unit cannot communicate with the Iridium satellites).

Since you have substantial misgivings, you may be better off with a different model, such as the GPSMAP 66i. It's a bit heavier, but it has a lot more features. It is the first iR model which is a full member of the Garmin ecosystem (just like other Garmin handheld GPS units). IMO, you probably want to stay away from what I call the "legacy" units (Explorer+/SE+). Those were designed by DeLorme before Garmin bought them. They are not members of the Garmin ecosystem (for example, they cannot use Garmin maps, the do not have ANT+ radios to pair with wearables such as fenix watches, they cannot use newer Garmin mobile apps such as Explore). The Mini has one foot in the Garmin universe and one in the Explore universe - not a full member of the Garmin ecosystem either.

Thank you for this detailed answer. I am much more interested in the SOS and weather features than tracking, so I wouldn't have the issue with the extensive power consumption due to failed tracking messages, this is important to know. I use my cellphone for navigation, so I don't need fast gps acquisition either, as long as I can get weather reports and SOS.

As far as the testing goes, if I switch the unit on, let it soak outside for 20m, switch it off, then I should get a fix in a few minutes. Sending a test message should be quick as well. These things are both easy to verify. 

I wasn't aware of the 66i, this looks like an interesting device. I'll have to check it out in a store. Thank you!

When you're testing your device at home it's useful to look at a map of Iridium satellites active coverage:  https://www.gsattrack.com/home/iridiumnextsatellites (ignore red circles, they are for inactive satellites). Using the map you can figure out when there will be a satellite in a part of sky that not obstructed by any buildings . You even can stay inside in front of a window if you know there is a satellite right in front of you and it will take a minute to send messages or update weather otherwise it may take hours. The map also helps to understand how the Iridium satellites are moving around the globe and how for example a cliff or a canyon will obstruct connectivity depends on it's location and direction.