Miller does some things well and others, not so well.
Does a good job of pressuring his opponent, has decent punch output for his size and seems to have decent punch resistance. On the other side though, he is too much of a stationary target and the less said about his head movement the better.
As has been mentioned in this thread already, guys like Wilder and Joshua would blast him away. Joshua would KO this guy in under 6 rounds. Even Fury would probably stop him and have him looking terrible in the ring.
A good test for him would be to fight the winner of Chisora v. Whyte which would give a better indication of where he really stands in the heavyweight division.