Flash is a major balance issue.
I have played this game for a long time and I firmly believe that flash is very unhealthy for this game. It is the single thing that caused the " league of mobility" as we call it. I think that riot realized this too with the introduction of the "grounded" debuff on
which stops the use of mobility spells. This is definitely a start but I think we need to take a step back and look at how flash is used about 97% of the time. If this was true about any other option then that option would be considered a problem. Flash is so essential that the grounded debuff isn't going to stop people from using it. They will be screwed without it anyway.