My 16 year old cat does not have a collar because I don't consider him a "flight risk"- he loves to sit at the window and look out but he has NEVER snuck out.
My two kittens wear breakaway collars with ID (and they are microchipped). They are so incredibly sneaky and fast! I have worked a lot on shooing them away from the doors to dissuade them, but they still dart out at times. One of them has taken her collar off a couple of times, but for the most part the collars stay on (although I have no doubt they could easily slip out or unclip them if they got stuck on something). Instead of tags that hang down I have the flat kind that rivets.
My dog only wears a collar when in our fenced yard or outside- not in the house. His collar is not breakaway and I have heard of dogs getting their collars stuck on their crate, getting an arm stuck in, etc. and being strangled. Plus he is trained much better than my cats. Of course he is microchipped as well.