I like my super feats... but there are always some that are just 'too much'. I remember Morrison I think writing a Nuclear bomb going off, and Flash evacuating every living soul in the entire city in less than a second or something. That broke believability for me. At that point there's not much challenge for a character like that.
Yep!! The mechanic I remember seeing was 'If you break the speed of light, you go back to your original time'. So literally EVERY time that Barry or Reverse Flash were bouncing around the future, someone was 'Hitting the speed of light' so he could go back home. There were situations like Barry being trapped in a cell too small to get up to that speed and things like that, but there were also times he had to 'run through space' in a Superman race where Speed of light was nothing and they were zooming between planets easy.