I think it's a matter of network performance, and the visual bugs that you can create without due care in such a fast-paced online game.
Valve have commented on this hero's mechanics, and issued fixes relating to client/server-side interactions within the past year in fact. But apparently, the exploiuts still exist.
Their comments have been around how redoing such mechanics can cause undue lag between clients that need to be 'caught up with' as quickly as possible. Too long and a client could see something they shouldn't and get a competitive advantage. Valve don't want to have a server updating 10 clients on 10 different network connections if they can avoid it. The compromise is to do more client-side, but that leaves the system vulnerable to such exploits.
Considering that the game includes fog of war, and invisibility mechanics, visual bugs from these systems do crop up a fair bit.