I don't really pvp much but does miasma not work in pvp or something?

The heavy effect from miasma paired with the fact you get to use sprint and have swiftcast makes it seem like you're not trying very hard to get away. The ninja movement speed bonus is nowhere near as effective as sprint and if a ninja uses sprint they lose all their stopping power and you should totally obliterate them.

This game is designed around PvE so I find it unlikely that you'll ever see true balance when it comes to PvP.