Delphi-PRAXiS - Don't mix objects and interfaces

Mixing objects and interfaces in classic (non-ARC) Delphi is a huge no-no. That part is clear. What that actually means is more of a blur. After all there is a object instance lying behind every interface reference.

So what is wrong with mixing objects and interfaces?
<ol>object references are weak, interface references are strong
reference counted object instances need at least one strong reference to keep them alive therefore you can't use object reference as primary owning re