I have heard the 'mic' units built into guitars and have never believed they were better than the Fishman or Baggs set-ups. They may be as good,but then I have only heard a few and so much of the sound quality depends upon the instrument and the player ....

I always wondered too, if the internal 'mic' set-up didn;t suffer the same problems as on stage instrument mics, namely feedback and EQ issues.