I’ve got an Acer ER491CR (49” curved). I’ve found the head / eye tracking to be fine across the monitor. The only issue I had I’ve had is mounting the Tobii unit.
The monitor has a little “pod” on the bottom where the monitor controls are (5 buttons.) The Tobii “under bezel” mount has two adhesive pads that need to stick to the bottom of the bezel, but can’t.
Thankfully one of the nice folks on the Tobii discord told me about a pinned post - this is a solution for folks like me that can’t mount the Eye Tracker 5 with the provided mounting tools. It’s a clip (you need two, of course) that grips the Eye Tracker 5 wider out, so you can avoid the monitor control pod. Whomever created the clips has provided the files for free, so you just need to find someone with a 3D printer or a service online to print them for you. It was suggested to them hot glue the clips to the underside of the monitor, and then attach the Eye Tracker 5.
Lol right now I have mine double-sided taped to a box in front of the monitor. 
Here’s the link to the clips in case anyone else needs them.