What happens when you need to repair or replace the roof? Standard roofs around here need a second layer of shingles after 10 or so years (don't quote me on the numbers), and both layers removed and replaced with a new single layer after another 10 - so approx. twice within the lifespan of the panels.

I'd expect some increased costs from the roofers, and a chance they'd botch the reinstallation of the solar panels etc.