I would say for the 24/7 activity monitoring, if anything, the fenix would be more accurate. However, for actual exercise, it would depend on the type of exercise/activity. For running, walking, etc, the fenix would do just fine, but for other activities, rowing, weights, zumba, etc, neither would be called "good", and the real only way to get accurate HR measurements would be with a chest strap. My opinion, your mileage may vary.
I already knew that chest strap would be most accurate, but I was talking about Mio Link and if there is any accuracy difference between Mio link and the internal heart rate monitoring of the Fenix. I used to do exercise with Mio link paired with my mobile phone, but now that I will be getting Garmin Fenix 5 plus soon, I am still wondering if I should pair my old Mio Link with it or let the Fenix do everything.