Difference between insert and milled putter

Many (probably most) insert putters are also 'milled'! Certainly all the ones I've used have been.

And some of the inserts (if they are metal) are milled also! Again, I've used plenty of those.

So this discussion shouldn't be so much about 'milled versus insert' as 'insert vs non-insert' imo.

That's not to say that 'deeply milled/grooved' putters like Yes or Rife don't act differently again!