If you were to take a rucksack I'd have to recommend the Kriega lineup. They make 15, 20, 25, 30 and 35 litre options. I personally own the 15 and 25, and have used the 20 a lot previously. They are all super comfy, even over long distances. I wore my 15 litre pack around Europe for a week earlier this summer without issue, but it was mostly just to carry my laptop (very light). I also use it as a Camelbak as it has a built in water reservoir.
There are a couple of things though. Firstly, crashing means you'd likely hurt yourself more with something attached to your back as mentioned before. Also, if it's a full pack and you have a tailback of any kind you will likely find the two touch and it can force you into uncomfortable positions on the bike. Thirdly, no matter how amazing the Kriega packs are at making the weight disappear, eventually it will get to most people.
Each to their own, but I try to avoid it whenever I can for long distances.
In terms of security, I like packs I can strap to the bike so well people look at them and go "I don't get how to steal it, so it's not worth it". I've left all my stuff on my bike all over Europe and the UK (cities, countryside, all over) without issue so far.