One basic problem I have with postmillennialism is that it not only assumes that the world is going to become better and better, more and more Christian until at last it is near 100% Christian, but it REQUIRES such a condition to exist in order for the Lord to return. The "all enemies must be put under my feet" idea. In addition to a lot of other reasons to question postmillennialism based on Scriptural and theological grounds, I do not see the world linearly, unidirectionally progressing over the last 2000 years toward a 100% Christian world, with total peace and complete obedience to God and the goodness of mankind. I'd like to see that chart posted in the safe house you refer to, if that somehow demonstrates this progress to a 100% Christian world. Cause the world I see is heading in the complete opposite direction. Before someone attacks me for suggesting the Gospel has failed, it is not God's fault but man that is failing the Gospel, as man has done since the hiss of the snake was first heard in the garden. Even though we now have the Holy Spirit in the world since the resurrection and Pentecost, we still continue to live in a sinful world, darkening by the day it seems.
Another big flaw I see in Postmillennialism / Preterism is the fact that the Lord did not return in AD 70 as described in the Olivet Discourse (Matthew 24:30-31) and in Revelation 19:11-21. Where in Scripture does it state Jesus returned in John's lifetime? Where does Josephesus say he saw anything resembling the return of the Lord as described in Matthew 24? If Jesus came again in AD 70, why is he coming back again? Where in Scripture does it describe a return in John's life AND another return in the future, or anywhere else for that matter?
Amillennialism has a big advantage over postmillennialism because it does not require this long period of near or actual perfect obedience to Christ, and the Lord can come back whenever the Father tells Him to, not dependent on human progress. However, I'm still a premillennialist simply because it seems to be the best fit to Scripture, though no eschatological system is perfect and without weaknesses, which is why you'll never hear me say I'm 100% sure I'm right.