I used to take the Montreal metro to work just carrying my laptop. Someone I kmow from a big European city was very surprised and said nobody would do that there, it would make you a target for theft. Here I wouldn't think twice about it.
Depends. Some parts of Berlin can be a bit dodgy. I take the U8 regularly Generally it's safe but stations on that line like Kottbuser Tor have a reputation for theft, pick pocketing, robbery, etc. Lots of people desperate enough in that area that you don't want to tempt them too much by being an obvious target.
I use a weather proof backback (Ortlieb). I got it two years ago and it's great. I walk a lot around town with it and the hip belt is great for offloading most of the weight from my shoulders. I can easily walk for a few hours with this thing. I have the 16" macbook pro, which is not exactly light.
You will learn to love the Ortlieb backpack. I bought mine (comparable to the current commuter backpack) more than 30 years ago and it is still in great condition.
That isn't a problem at all for most Western/Northern European cities though, without naming it makes it quite hard to judge if it was sensible advice or anxiety.
I live in Edmonton, a friend of mine from London was astonished that I'd leave my bag by my chair unattended when using the bathroom at a pub. I don't even think about it, my stuff has never been nicked.
The same thing done with stolen iOS devices; end up in Shenzen or Varanasi in a "might-be-used-for-parts" bin., until sufficient harrasment/social engineering of the original owner has failed. Then it goes into the "used-for-parts" bin.
