Is building Qi in the dantian really safe?

You may have heard stories of these self taught Mopai guys getting all sorts of physical problems from trying to create and store large quantities of Qi in their dantien. Cancer, Bladder meridian problems, prostate problems, headaches, etc. Let me assure you its very possible to get these types of problems especially if you try to practice neigong without a teacher who knows what he is doing. Whats less well known is that even with a teachers guidance depending on your body constitution and previous issues you can still develop these problems. It seems that building qi in the lower dantian can benefit some types of problems but on the other hand cause more harm in others. Ive seen students from schools such Mopai and Gengmenpai develop such issues. I also seen students from Longmenpai develop other types of issues unrelated to producing large amounts of qi in the lower dantian. The exact reasons for the developing of such problems is unclear. It likely has to do with having preexisting Qi/body constitution issues and not having regular access to go to get progress checked. In the old days students of neigong would have regular (weekly/monthly) access to their teacher who could prescribe the proper herbs, lifestyle choices and changes in practice necessary to counter such issues.