The rack is attached at two points to the top of the fender with M5 bolts. The fenders are stainless steel with attachments at three points: the fender stays, a clip at the brake bridge, and the chain-stay bridge. It's difficult to push down on the fender itself. Adding the rack which has its own vertical struts has only made the whole assembly even more rigid.

I'd might be more wary with lightweight aluminum fenders. But with the stainless steel ones, the rack is rather solid.

The rack is really meant to hold small panniers and to the steady the bottom of a saddlebag hung from saddle loops. I wouldn't trust it with more than 40lb capacity.

I'm installing a fender mounted light sometime this month and can post picture of the inside of the fender & the whole rack setup if you'd like.