Vertical punchers say striking with first two knuckles is dangerous. Some posit that, even though striking with the first two knuckles concentrates the power of the punch in a small area to cause more damage to the target, the concentrated forces would also damage the knuckles. If this were true, then there would be more fractured hands during board breaks than broken boards.
Stand in front of a wall, get into your fighting stance with clinched fists, extend your lead arm in horizontal punch, and move forward until the fist makes contact with the wall. Usually the middle knuckle makes contact first, with a slight adjustment the first and second knuckles make contact. If you rotate the punch to a vertical fist, the first and second knuckles make contact. The only way the last three knuckles make contact is for you to angle the wrist consciously.
Any supposed benefits gained from striking with the last three knuckles are negated by the bent wrist. To change the striking point of the punch to the last three knuckles, you must bend the wrist toward the thumb side. With the wrist bent, the line of force from the last three knuckles is again in line with the wrist and forearm, but it takes a conscious effort to bend the wrist and keep it bent. If the fist misses its target slightly so the first two knuckles strike the target, the bent wrist will put a large bend in the line of force through the wrist that may result in an injury to the wrist. Since it takes a conscious effort to keep the wrist bent in the vertical punch, any lapse in concentration will allow the angle of the bend to decrease, resulting in something between a vertical and a horizontal punch, which will probably be useless.