The response that those of us who inquired during the Baldacci administration received was that several states would not honor a Maine permit unless or until Maine's statutes are revised to explicitly codify training and background check requirements.
As it was told then, the opinion of the other states is that Maine's statute has too much wiggle room for the local issuing authority to waive training and/or to not use a Federal database for a background check.
The other side of the coin is that Maine refused to accept permits from states that didn't allow the issuing authority to evaluate "moral worthiness" based on anything other than criminal convictions.