Most of the cases I see involve people running down lanes with no items, or even worse, buying them and dropping them (wards), so these are always found guilty.
The "non guilty" cases seem like normal gameplay, jus that the player is obvious noob (I assume 100 hours per rank) and the builds seem nothing out of normal.
Regarding "insufficient evidence", these are cases with very little amount of game time, and I do not wanna waste time by searching for the match replay (given the score is usually 0-5-1, nothing like 0-40-0).
The only interesting case I have seen in a long time was Shadow Fiend reported by Tiny Airlines for feeding (unranked, no roles). Previously, there seemed to be more of these. People farming jungle for 7th item instead of finishing, different versions of "shadow amulet/treeline blink" , following somebody when farming…
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As I currently now have access only to 3rd medal and unranked matches, and given I am recalibrating (do not wanna play ranked now), Valve might not want me to handle actually questionable cases – could that be the reason? Or did simply people give up on creative griefing? Or reporting it? Because if the last one is the case, it means Overwatch is not really doing its job…
Source: https://www.reddit.com/r/DotA2/comments/uvlxni/i_am_not_getting_any_interesting_overwatch_cases/