I've always used a Garmin for my outdoor activities, started out with the Fenix 3HR and now using a Fenix 5x. I have a vivoSmart HR to wear everyday (on my ankle), for step count.
Outdoor activities that I do and track are trail running, indoor bouldering/climbing, hiking, indoor rowing and strength training at the gym.
What I like about it is that it can track various activities, specific activities too. If it's not loaded on the watch, I can download the app and easily install it on my watch (e.g. StairStepper). Battery life is good enough for me, it is rated for 12 days if you just use the smartwatch function 20 hours in GPS mode. I can go on a trip and use my watch to track outdoor activities without having to charge daily or every other day. It comes with a Trackback feature, if you get lost, just go to that and it will re route you back to the starting point. I will tell you I used this once and it did not work for me, it was my fault , I was not reading the instructions correctly..also continuing to run while lost on an unfamiliar trail (granted it was Pre's trail, which some may say is impossible to get lost on) is not a smart idea.
It was heart rate tracking, and can tell you how much time you need to recover in between activities, which heart zones you're hitting, pace, cadence, elevation gain, how productive your training is (optimal, overreaching) etc. The watch is preloaded with full color TOPO US map, i have been able to load gpx files without any headaches. The watch can tell you what's nearby locations are around you and it has a feature that allows the watch to map a run for you, just put the distance in and other parameters you prefer and it will route you (haven't tried this one yet). When strength training at the gym it's supposed to auto detect the activity, i.e. lat pull, pull up, squat etc. It has not been very accurate at all, out of 14 reps it can maybe identify 3 or 4 and miscount the actual reps (no i cannot quite do 34 pulls up yet Garmin
Smartwatch features -- it can display notifications from your phone i.e. calls, text, Twitter, Instagram etc. It does not work as a speakerphone like an Apple Watch to take phone calls, you can either reject or accept phone calls but the calls still go through the cellphone. I find the Music app very helpful, from the watch I can control audio on my phone, important when I use truly wireless earbuds that does not have audio controls when used independently (i.e. Jabra Sport Elite).
Their Garmin Connect app is pretty easy to navigate. Gives a nice clean visual of what your day looks like, your goals, has insights to see how you're doing, you can browse watch apps and my favorite, you get to connect with other Garmin Connect users (there are a handful of AZ groups).
What I do not like is the size. I get it, the screen has to be a certain size for the map and data screens but my petite wrist is overwhelmed by the size of the case. There is a smaller case size but the features are different from what i can recall.
It's comfortable enough to where it does not annoy me when I wear it during activities but not comfortable enough to wear as a daily watch (weird i know). That's the only thing that i have qualms about for the Fenix 5x.
If you are looking into the Fenix line, they just released the Fenix 5x Plus a few days ago. I haven't looked into it yet but I do know that it can act as a Pulse Ox. I don't know that I want to know my O2 sat when I'm trying not to lose my mind on an exposed class 3 hike at 14,000 feet. j./k