I had a real inexpensive tach(Tiny Tach) for my Rebel some time ago. I think I paid $35.00 for it and used it to set the idle. I now own a beautiful VLX 600, 2005, and would like a simple and cheap tach to set the idele.
My tach is still working fine after several months.
I had wondered about the sun myself since some LCD readouts are more sensitive to others about being exposed to direct sunlight, and some will just turn solid black until they are shaded and allowed to cool off, but so far it's handled it OK.
As far as rain, I haven't ridden it in or let it sit out in much more than a light rain, and it didn't seem to take on water. The front panel is a single piece of clear flexible plastic so there are no places for water to enter other than the outside edges of the plastic sheet.
The only thing I don't like is the one second update interval which is not as fast as I'd like, but is OK for checking rpm at constant throttle settings.