I've towed both over and under GCWR, have felt unsafe both over and under, along with very safe both over and under GCWR.
I have generally speaking felt safe under the vehicles GVWR, as long as I was under both the front and rear axel capacities. If over an axel wt capacity, then things get goofy. I have also felt safe over gvwr, as long as I am under the axel wt capacities.
At the end of the day, as long as one is pulling speed wise what would be considered safe to an leo, braking per state rules, trailer is not uncontrollably swaying side to side, no bad handling from the trailer or tow rig, you are generally speaking going to be safe.
On the other hand, if the trailer sways side to side uncontrolled, it will not matter HOW MUCH under/over your ratings you are, you are in a very UNSAFE situation!
Marty