It alright to have a PTZ in the sun HOWEVER, the plastic housing will not last as long being in direct sunlight. It will start to discolor and need to be replaced.
I have aSD52C225U-HNI 2MP 25x Starlight PTZ on the front of my house now for 2 years. It gets direct sun for about 4-5 hours per day and still looks very clear. If you can I would recommend not using a dome camera outside not only for the fading, but the IR reflection can be pretty bad.