How good is the GPS and altimeter on the Fenix 6?

That's a simplistic vision of mountaineering.

Maybe he summited one sub-14er and mistook it for the right 14er. Especially if their height difference is not large, sometimes it's difficult judging whether a nearby mountain is higher or lower than your current location.

I am no expect but I have trouble understanding how getting to the wrong summit can be blamed on the GPS accuracy of the watch ... seriously summits are not 5m or even 25m apart. There must have been something else going on.

The barometric altimeter in Garmin watches are accurate (if calibrated correctly) and work up to 30,000 ft. Much higher than anybody on here will be climbing. But the watch has different modes of calibration and that's the important thing to get right.

Turn off auto calibration, turn off any sort of calibration before, during, after an activity.

Set the watch so that any pressure changes are considered to be a change in altitude.

DEM calibration can be pretty accurate depending upon the source of the data. DYOR.

GPS calibration is the least accurate, do not use it.

The most accurate form of calibration is from a known height, either use a map contour line, or a trig point, or a specified reference point of known altitude.

The trouble with using any barometric altimeter is that a pressure change will cause an altitude change whether you have gone up or down or not. So for general activities like a run or hike that might be an hour or so, you can usually discount pressure changes caused by weather, unless a weather change is approaching fast like a storm front.

If the activity is longer or in a region where weather changes are frequent such as a mountain region then as the pressure goes up or down your altitude reading will be affected.

To counteract this it is important to recalibrate your altimeter frequently if at all possible.

As an example, GA pilots altimeters are generally barometric and as you are flying around the country the pressure frequently changes, so the altimeter is recalibrated at every opportunity, can be every 10 mins or half an hour. Also as you join an airfield circuit you will be told the local pressure so you know accurately what your height above the field is.

There is no magic wand with altimeters, put them into the simplest mode you can, disable all the auto stuff, and recalibrate often, and they will be pretty accurate, about +/- 25 - 50 feet. That's about as good as they get, especially the tiny little ones you get in a watch.

I hope this helps a little.

You should absolutely NOT confuse GPS accuracy with a distance measure. You essentially use a paper map methodology with a GPS. The distance is far less accurate and depends on so many other things. One is what you compare against. How do you know that independant source was more accurate ? What did they base that distance on ? Was it a 3D distance or a flat distance ? A map base measurement will almost always significantly underestimate the actual hiking distance because the trais have many small twists and turns that the map does not capture but the GPS tracking does. There is also the effect of elevation while a map is flat the watch can take it into account. If you are serious about navigating learn to rather use waypoints and courses so you set a location-based references and not a distance based reference. Anyhow your issues are not related to the GPS accuracy but rather to your way of navigating. In your case if you had set a precise waypoint to your destination and tracked that with the watch you would have got right at it - as long as you set the waypoint accurately