Hard-style attitudes

Many tai chi self defence instructors have an external martial arts background.
This is valuable experience but also an impediment.
Tai chi approaches combat in a very different way to the hard-style arts.

If you apply external methodologies and tactics to tai chi, it simply will not work in self defence.
At best, you'll have an external parody of tai chi.
At worst, you'll simply be defeated. 

You cannot train external and internal arts simultaneously and hope for the internal to work.
Under pressure the external would come out, not the internal.