According to your own post you've never seen him do that anywhooo; so how would you know how low he hits... How low? Low enough to take out guys' knees.
Hip tackle has its risks, but IMO is cleaner than the more common tackles aimed to the opponents head. Virta´s injury was an accident and I have not heard him blaming Koistinen. Saying Koistinen a dirty player due to TWO incidents is quite an odd statement. I see those "victims" (Louhi and Virta) a lot dirtier than he is.