The panniers look great! Very slick-looking.

I would be a little worried about the fiberglass joints, too... the initial strength is probably fine, but it seems like there might be an issue over time, with those joints being load-bearing, and flexing repeatedly (especially if the miters aren't very precise, as you were saying). I imagine you could see hairline cracks forming long before they failed, though, if that really is an issue.

I'd love to hear how it's working for you over time. Good luck!