My first 2 kids were good sleepers. My 3rd child is an appalling sleeper, although he is in the 4 month sleep regression atm. Dealt with them all the same, no sleep training just cuddles, kisses and co-sleeping. I def believe it's personality. Not that they can't be smoothed out, I don't think a bad sleeper is always a bad sleeper. But I think the idea that a baby that doesn't sleep well is somehow the parent's fault is rubbish. If it was purely technique DS2 should be a good sleeper based on the other 2.
I personally don't hate on those that sleep train. It's just something that isn't right for me and never will be. There is a big difference between the two.