I just got the Apple Watch 5, and I'm considering putting my Garmin 645M in the drawer.
The GPS accuracy is better than it was in the Watch 2 (which I previously had prior to the 645M), but probably still not as good as the Garmin. However, for every other function, the Apple Watch is better. I usually run / ride the exact same routes, so I already know precisely how long they are, and really all I'm after is elapsed time, so I think the Apple will work for me.
If you need splits you have to stay with a dedicated running watch with actual buttons for start/stop/lap. The apple watch still has a big problem with recognizing sweaty finger touches.
Back to the GPS accuracy, this weekend my wife and I hiked 10 miles through woods and hills. She wore a Garmin Fenix and I wore the Apple 5. For the first 8 miles the distances were about as even as you could want. I had 8.01 when she had 8.00. Then over the next two miles, the Apple sort of started losing accuracy. Not sure why, maybe the tree cover was too dense, maybe there were too many switch backs. At the end she had 10.00 and I was at 9.95. Not a big enough deal for me to worry about, but in the past, but for some, it no doubt would be.