Scars can be more painful to get tattooed but most of the time it's about the same. As far as tattooing over scars, it depends on how fresh they are. If they're fairly fresh (red, raised, puffy) they can't be tattooed because they will not hold the ink well. If they're kind of old (more faded, not as raised), it shouldn't be an issue. Just take very good care of it and it'll be fine.
As far as the wrist comment above about it not healing well because of the wrinkling of the skin, that's not true. I have one arm completely covered (elbow area and all) and the other forearm covered, and there are no issues with healing and pulling ink out as long as you heal the tattoo properly and take good care of it. I work at as an artist at a tattoo shop and the main issue I see is when people don't take proper care of their tattoo. Like tanning or going out in the sun after a fresh tattoo with it exposed for extended periods of time, picking scabs, not applying lotion like they should, etc. Just follow aftercare and you'll be good with almost anywhere you put a tattoo!
