Well G have pretty much said they ignore toxic backlinks these days, and you should be able t sort out scrapers/dupe content with proper canonicals in the first place, so your clients can stop worrying :)
Reassuring folks works 3/4 of the time, the remainder usually hear/see a 'case study' where disavowing takes a site from zero to millions of visitors (just pay $999)
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The main thing my clients always worry about is bad inbound links, closely followed by scrapers causing duplicate content penalties.