16 tick makes it almost impossible to determine between legitimate and illegitimate aiming. The current AWP meta makes flick shots very commonplace, and visually there is almost no difference between an instantaneous aimlock, and a nasty flick. Same goes for flickshots with other guns such as AKs. Back to 16 tick, even recoil looks wrong, choppy, and strange, which is a huge issue. A very common facet of modern aimbots is automatic recoil compensation, which looks obviously suspicious on smooth demos, being inherently choppy, clicky, and mechanical. Problem being that they appear almost identical to legitimate sprays on 16 tick demos, allowing most well-coded aimbots to reign free without retribution, save for VAC.
Most troubling to me though is the 'other hacking' section, which is usually saved for Bhop hacks and the such. Big problem is that you can never rightfully tell if someone is either skillfull at bhopping, or truly scripting purely visually. Some are more obvious than others, such as the FrankieOnPC1080p debacle, but someone who hops like phoon could either be skilled or scripting. Only way to actually shed truth would be code-side, which routes right back to Valve.
Another problem is something I like to refer to as Omniscient Information Bias, being the fact that you the spectator know where everyone is, know where everyone is aiming, and know what everyone is doing. It can be quite easy to say one is hacking if you see them glance at another through smoke, seeing as you the OW investigator have X-Ray, but one has to be cognizant of the information the player had, and what the situation was. Say for example a smoke is through into T-Ramp on Mirage, and a T hears a step. If a T were to peek out, and headshot a waiting CT at close left through smoke, it would look damning from an x-ray POV, but not from a logical one. Watch some of your own demos, I can guarantee there will be some seriously fishy looking aims through walls, and through smoke traces, but those don't inherently indicate hacking as a cause.
The client and system is itself are also prone to bugs and errors. For example, I had a case just last week where the first time I watched the report, the Suspect shot a man directly through smoke, but upon postponing judgement and re-watching it, the smoke wasn't there, and the angles and scans seemed completely legitimate.