Navagation was the biggest problem.
No GPS, compass's were not that reliable. Getting a bearing from a radio beacon was not all that acurrate, and frequently jammed.
Weather was the worst they'd seen in years though much of the war.
So you take off, climb into clouds, (put on a blindfold) make a couple of turns, have wind direction change on you as your climbing.
So now your somewhere over Europe, doesn't matter HOW good your bombsight is if you can't pinpoint where you are.
On the big raids you had 500 - 1000 Planes, everyone supposed to drop on the leaders drop. You've got planes scattered over half the sky, gee, you think you may have bombs scattered accordingly?
Ever been up in a commercial jet?
Look over the side at 30,000 feet & tell me what you see. Now suppose you were flying over where you live.
Think you could pick out "your" block out of all those other blocks at 30k?
Its tougher than you think.