> Care to elaborate in what circumstances is using a "personal machine for work" a problem and why?

When you stop working for an employer/customer and you are legally required to purge all files.

Having everything work-related on a dedicated machine makes purging all files very easy.

Not having everything work-related on a dedicated machine makes purging all files questionable.

Ok, so one circumstance is when there is a "legal requirement to purge all files". That's certainly not "always" and not always as hard: one of my jobs was at an open source company, and there are plenty of those to go around these days; as a consultant, you may start with some base work to build off of that you keep the rights to as well...

This also assumes you never-ever used a personal device to access any of them either (they might be in caches or Trash/Recycle Bin) — and I agree that to satisfy such a legal requirement, you probably don't want to be using a personal device to access them at all.

Keeping things separate has some upsides, but also some downsides (multiple devices to lug around) — depending on their situation, everybody should choose their own compromise (granted, some engagement contracts will make that choice for you).