It's unfortunate that the demo system isn't accurrate enough to really get aimbotters in TF2 it's been talked about in many threads. A lot of cheaters use a "trigger" exploit that basically allows them to aim naturally then the exploit auto-fires when a character model crosses the target. It's unbelievably difficult to detect from spectating or through a demo there have been high level pro players caught doing this as well.

Considering the age of the game it's just something that will happen from time to time I don't disagree that there might be some players who use it and also some players using materials hacks to see through walls. A major redesign of the game is required to resolve the exploits and Valve will never do it at this point. Just keep an eye out, record steamIDs and demos, and if enough evidence is available they'll be banned.