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

my Fenix 6 altimeter is dead on accurate, perhaps you're not aware of the fact that at very high altitudes barometric altimeters are characteristically highly inaccurate due to the way they're designed, but if you learn how to compensate for it they can be extremely accurate.

A little math to set the stage, baro altimeters are all designed to a pressure/altitude curve that is based on what is called the "standard atmosphere".  The standard atmosphere is defined to have an air temperature of 15C at sea level, and decrease by 6.5C for every 1000m of altitude.  So, based on that, at 14,000 ft (4267m) the expected temperature is -12.7C, let's call it -13C.  Its quite possible on a summer day for the actual temperature you're hiking in to be something like 10-15C.  If its 15C, then the altimeter will read incorrectly by the proportion of the actual temperature to the expected temperature, with both temperatures in Kelvin (C+273).  So, 260/288, or about 10%.  What's happening here is the baro altimeter expects the air to be much denser than it actually is, because the standard atmosphere curve says the air should be much colder (denser) than it actually is.  In this case, for every 1000ft of elevation change you hike, the altimeter will read 900ft of change.  

Would this explain what you were seeing?

FWIW, after 35 years of doing this math in my head, I wrote an app for my Fenix 6 that does this calculation for me.  I've hiked with it at near sea level, and at about 10,000ft, and it worked perfectly in both instances.  By perfectly I mean after gaining/losing a few hundred feet, it was literally within a few ft (1, 2 or 3) at several known altitudes based on Google earth and topo maps.

In my experience, altimeter is pretty good, thanks to continuous calibration using DEM (digital elevation map).

However, GPS is pretty mediocre, likely worse than in much cheaper Garmin watches.

I have no problems with my F6 and would highly recommend it. 

Please remember that the people with problems on this forum do not represent the majority of users... even though some of them seem to think they do. 

GPS good for me, but altimeter not good, sometimes up to 500 meters wrong, and almost never correct, usualy in +-300 m range

I was not aware of the altitude issue.  Indeed, I was hiking in CO at some altitude.  But my Garmin 66i is pretty accurate.  Why would that be?

I have a Garmin Instinct.  Same GPS issue.  I have read that pulling off accurate GPS with a watch on your wrist is a challenge.  One review attached the Felix 6 to a pole and the GPS worked well.  I may try my Instinct attached to my backpack somewhere.

In my experience every baro altimeter will follow the standard atmosphere curve fairly accurately, so if one is off, others will likely be off by the same amount.  Were you using it on the same hike as your Fenix5 and it was reading correctly?

Another variable I didn't mention earlier because I don't use it, people here have claimed that having the auto calibration mode turned on with the Fenix 6 often produces bad results.  this comment is also directed at matej787.  they have also claimed that the DEM database isn't so great in mountainous regions.

Yes, used both at the same time and compared them.  As I noted, the GPS on the Fenix 5 was pretty useless, though that might be explained by the fact that it was on my wrist, the 66i, on the other hand usually had the antenna pointed to the sky.  That wouldn’t explain the poor altimeter readings on the 5, though.

I have owned an MK1 (F5 based) and had this kind of altitude issue on almost all alpine activities. That's the main reason I have sold the MK1 and bought me an F6Xsolar 14 month ago. For me, this watch is great for all alpine sports, typically the barometric based altitude is within +/- 10m. For curiosity I have tested the different calibration methods in alpine terrain. By far worst was DEM, I guess this due to a combination of a limited field of view to the sky combined with the rather low resolution of the DEM model used. My personal best practices, I calibrate the altimeter at the begin of a tour and let the altimeter do the rest of the job.

Fenix 6 Pro: very happy with the GPS and the altimeter. I use them everyday for my job.
Not happy with the compass... totally unreliable.