We never use fly masks, for the reasons stated below:
Unless the fly mask is a PERFECT, custom made fit, flies can and do get inside the mask and can feast on your horse. We've had this happen many times. Even a small opening, they got inside and bit the horses relentlessly. The flies can get in a small area, so to keep them out, the fly mask would have to be form fitting with not even the slightest gap .
When the horse itches/rubs whether due to itchiness or in an attempt to get the mask off, the mesh can rub the hair off around their eyes, and in some cases, could damage the eyelid and/or cornea as even a soft mesh is abrasive. Some manufacturers state that one must be careful to make sure the mask fits loosely over the eye....but even so, rubbing makes that advice null and void. If a horse rubs their face against something, the mesh becomes a brillo pad...
It is a danger when pastured with an aggressive pasture mate, as Beau is.
We don't use them, and we have very little problems with flies on the face/eyes. We simply use a roll on/wipe on fly repellent like Endure roll on for their ears and around the eye area....and that seems to work wonders. Haven't used fly masks for years.