When i replaced mine, I screwed mine onto the front and used sticky pads on the back. As someone said, the front is curved. And tbh, it's easier to take them off with screws than it is pads. I had a bit of a mare trying to get the old plates off because they were stuck down so well.
iirc - There's screw holes in the rear bumper for plates anyway.
I wouldn't feel comfortable with velcro personally.