While yes, all laptop may have a seam, the problem here on framework is much more relevant due to its design. Most of the ultrabooks use downward-facing motherboard and are opened from the bottom cover, so they may well hide the seams under the bottom, which users do not usually touch or see. FW however opens from the keyboard side, for which the seam can be pretty obvious and large seams will be much more of an issue when touched.
And no, there will not likely be any more sharp edge or reduced ability to open if the keyboard and bottom cover can be precisely CNC’d. And the seams are supposed to be much smaller than aluminium forming.
It would also be good to have 6063 aluminium alloy than the 5000 on the keyboard plate and the bottom cover. It will help with the issue that the keyboard plate is slightly weaker and can be bent when more force being applied along the center of the plate.