The biggest reason I separate work and personal devices is that I do not want my personal devices subject to discovery if my employer is involved in a lawsuit. I can't stress this enough- if my employer is sued, I don't want the opposing law firm to get copies of my data.
The second reason is that I don't want my personal devices subject to security monitoring, and most companies that at least pretend to care about security require some level of endpoint monitoring.
I don't know where tedivm works, but their concerns are reasonable for almost any industry.
Small companies usually can't be bothered worrying about those risks (and small companies aren't typically worth suing anyway). But a company of any appreciable size needs to worry about the kinds of things mentioned in the comment.
The second reason is that I don't want my personal devices subject to security monitoring, and most companies that at least pretend to care about security require some level of endpoint monitoring.