I have a TPO roof. We have 420W of solar on the roof and are adding 160W more. No problems. I like them attached to my roof because:
1) I'm lazy and wouldn't bother to set up the portables half the time.
2) sometimes we park in places where setting up portable solar would be difficult (parking lots, rest areas, etc)
3) I'm afraid of them growing feet
4) less obstructions on my roof to worry about than there could be on the ground so I don't have to worry about where the sun is as much.
5) on my roof, they are always working rather than taking up valuable storage space.