I have a Polar Vantage V which I have been happy with for use in triathlon. For ultra trail running it’s not up for the job. I struggle to navigate and always run of track a few times too many.
Trail runners seem to love Garmin Fenix, but I find it big and bulky for triathlon.
The Garmin Enduro 3 looks better in terms of weight and size. It’s highest on my list for now, but sure comes at a solid price.