I posted this as a reply in the 935 forum a couple of days ago, but thought it could be of interest here as well, since my device is a Fenix 5.
This summer my F5 completely froze when navigating a downloaded route during an MTB session. The route had far fewer data points than what I have understood is the upper limit. I downloaded a 60+ km MTB route from Garmin Connect, and used it on a session. Everything worked fine initially. I didn't even start at the starting point, but was given directions (properly) as I approached the route. As I rode the route I was given turn by turn information properly, but after some 30 to 35 km the map screens started to lag somewhat, and getting slower and slower to update/change screen as a continued. At 45 km the screen stopped updating totally, and the watch was frozen. I tried all buttons without success, and tried a soft reset (long press light button), but just got "the triangle" back.
At this time I was at an unknow trail... and could have used the navigation. (I did not know exactly where I was... and going backwards was out of the question). It took some time to complete the route on unknown trails using the sun as an indicator of direction... Only a later master reset woke the watch up, and the bike session plus history plus all settings was lost. Although true up did re-load my VO2-max and training load.
Up till then (15-16 months of use) I had not experienced any major problems.
Now my watch did freeze again this week, whilst doing a strength session using reps counting. This time a soft reset re-started the watch and the session (but I lost accumulated training effect). I may add, that yesterday I created a route using EasyRoute, that I downloaded as a GPX-file via Garmin Connect to my F5 for a shorter bike session. This time everything worked as expected... unless the freeze today was a consequence of me running a route in the F5 again, although the freeze lagging almost 24h.
I realise that it is difficult to avoid bugs, but navigation is one of the properties that I'd like to be able to trust. The MTB bike session turned into and unplanned exploration session, with me being an hour late...
Will go hiking in the Alps shortly... but will most certainly use traditional tools, like a map and compass... A bit disappointed because of this... otherwise I have logged plenty of hours, and the F5 have up till now been more reliable than my earlier Suunto Ambit, and my Polar RS800 before that.
