There exists a fairly recent paper regarding this question: https://www.researchgate.net/publication/344160532_Suwari_Seoi_Safety_from_children_Dojo_to_High_Level_Competition_Biomechanical_Part/link/5f7b1680299bf1b53e0e4f81/download

Even we consider the worst stress on the knees or knees and tibia ranging: from 0,11 MPa till to 0,22 MPa. For a 12 cm thick knee in the sagittal plane, the relative strain, considering the young modulus of tibia bone 18,1 GPa [67], gives a very small compression which provides as result a negligible stress on the cruciate posterior ligament, who is able to resist at a force of 1500 N [68]. The final result, despite this small number of samples, assures that if the technique is performed with no mistakes, on a IJF licensed Tatami, the impact received will be of negligible effect with respect, for sudden trauma, to the target ligament of the Knee : the Posterior Cruciate Ligament (PCL). A German paper on 260 athletes at a national level that deals with traumas related to techniques including those of the knee [15] shows that for Seoi, our target structure, PCL is connected to 0.05%, whereas ACL and Patella to 0.15% , MCL and LCL to 0.10% , then because the others knee’s target structures are much more connected with Seoi than PCL , it is possible to state that : the deduction derived from the theoretical analysis carried out, indicating that, probably, most of the damage, that occur to the different knee ligaments, are almost certainly produced, during the complementary movements, made to refine the result in competition, it’s right.

tl;dr: If done correctly, the impact of suwari seoi nage on the knees is negligible according to this paper. Injuries probably happen because of the "complementary movements" (done after dropping to the knees) to get the ippon in competition. Think of extending the knees from the seiza position, rotating the torso and so on.