My switch to SPD M520's came from my mountain biking days. My shins and calves have the pedal bite marks to prove it. Get better pedals and shoes they said, get something pinned and a grippier shoe they said. I opted to go clipless, and have not looked back since. Riding fixed means you need retention of some sort, so SPD's were a no brainer. I have them on my fat bike as well, as I have grown accustomed to clipping in and out.

Adjusting to them was fairly easy. You may forget once that you are clipped in, but that is usually all it takes. Unclipping becomes second nature, and when I was mountain biking and wrecking, it was never because I was clipped in. I also managed to unclip every single time. Your brain figures it out quickly, and your muscles learn as well. It is no different than riding a bike to be honest. You don't have to think about pedaling, you just do it. Same when you go to stop and you naturally unclip.