I have been managing digital campaigns since 2004 and I have now done forensics on 100's campaigns. In 99% of them there was waste from 20% - 80%. The main culprit is poor placements on publishers that are probably not right for you. You combine that with a weak attribution model or the wrong KPIs and you get waste.
Want to check for waste in your campaigns? On google run a placement report and look at mobile apps as well as publishers with high CTRs. On Facebook do a breakdown report by platform and look for audience network. For most advertisers you will be better off redeploying that spend.
It goes far deeper than this, but in general you should be cherry picking platforms, not selecting all on them.
