Throw ynab-buddy at any virusscanner you want. They'll tell you it's safe. Click here to see ynab-buddy's results on VirusTotal
Because ynab-buddy hasn't been signed by Microsoft. This is their new way to 'fight' malware: either a developer pays around $100/y to get their code signed, or the users will see this warning.
Since this is an open source project I'm working on in my free time, I simply didn't want to budget for this 🤷♂️.