When to walk away is a personal appraisal, I have done a trapout up on a roof top, and would do it again. Here are some of the most important aspects -
1. Take into account the fact that the hive body will be MUCH heavier coming down the ladder than it was going up. It is hard to bring a hive body full of bees down a ladder, and you don't want to disassemble it to do so.
2. Will your setup survive a storm?
3. Can you reach all the areas that the bees may be exiting the site, so you can reduce the exit to one spot? Is that spot able to be closed off with the funnel with a box ajoining, or is it in a funky corner or something?
4. Will the bees endanger anyone at the trapout site? They will be upset, and there will be A LOT of them at first.
Just some thoughts.
JC