1. No real downside, but you should think of them as permanent (see point 5)
2. Depends on the brand, quality, and thickness
3. Depending on the brand and quality, they can last from 5-7 yrs in the elements, longer if garaged.
4. When they hit the end of their life, than can start to peel and crack. At that point, remove, clean any adhesive and re-apply
5. They do not harm the paint. In fact it protects the paint. If you were to remove vinyl stripes after 4-5 years, you would notice the paint under the stripes as being perfect, but the paint outside the stripes would show natural wear and fading