Over the years studying the 10th Planet system, the half guard game has been the most valuable to me. As such, I feel very comfortable when I have someone in it. In my experience seeing literally hundreds of people use it, the lockdown can and will get the tap on some people. I have seen the lockdown also cause knee damage on about 6 people over the years.
The pressure from the lockdow can be too much for noobs or guys who are not use to that sort of half guard. Then, there are guys with incredible crushing power from the lockdown than just want to make you tap. Some guys have shins like blades that cut into the calf muscle.
I would say it is rare to get a tap from the lockdown but NOT impossible. It is even more rare (but not impossible again) to actually damage the knee joint from doing the lockdown.