Doing them on my knees just means I had to start at a slightly lower weight. I don't really mind it anymore, I just needed to get used to it. It doesn't hurt at all, I kneel on a rubber pad. I would like to experience the hip drive up that standing allows, but I can always learn that motion later with no loss of strength in the meantime. In a way, it makes the press hit my shoulders even more since they have to get the weight started. I do hip drive on my knees too that is smaller but still helps. Better than paying for a gym just to do one exercise slightly better imho.
Edit: When I say hip drive, I mean the horizontal swaying that is part of a proper press to get the weight started moving, not a vertical drive.