I used a GloBee for three maybe four years and it worked fine, it broke, (or some might suggest that I killed it by leaving it in the sun and ruining the LCD).
It was replaced with a Tower Hobbies branded tach and it works fine too.
Since you specifically mention bright sunlight causing a problem maybe orientation of the tach relative to the background is causing a problem.
When the sun is behind me I have to make sure that the sky is the background that the tach "sees" instead of the ground.