It's in the original report, which several other people have already linked to:

https://adalytics.io/blog/invalid-google-video-partner-truev...

There's a detailed methodology section. I haven't verified their results, but they report finding things like muted, autoplaying ads even on big-name sites like the New York Times.

A few key quotes:

"For one major infrastructure brand approximately 73% of their TrueView skippable in-stream budget was spent on Google Video Partner sites or apps whose ad delivery does not appear to be consistent with the TrueView or in-stream requirements."

"A major consumer goods brand had 75% of their TrueView budget delivered against Google Video Partner (GVP) sites or apps. 56% of their total TrueView budget was spent against websites which served TrueView ads in muted or non-in-stream video players [...] 19% of their TrueView skippable in-stream ad budget delivered against ineligible mobile apps which are not video streaming apps[...]"

"A Fortune 500 brand had approximately 57% of their total TrueView in-stream ad budget served on GVP sites and apps. 35% of their total TrueView budget was spent against websites which served TrueView ads in muted or non-in-stream video players [...] 8% of their TrueView skippable in-stream ad budget delivered against in-eligible mobile apps which are not video streaming apps."

I'm no marketing expert, but 43-73% of one's ad budget basically being thrown away seems like not great results.