mana burn is fine learn to play
You do realize OP that the cast is 2+ seconds long...Any team that is 'good' will easily see it going off and interrupt/silence it. I rarely even use it anymore.
The only time when I use manaburn or mc vs melee equivelants in an arena is to force tje enemy to either LoS or interrupt me, lets me get a cheap heal off. People who eat through the cast time, well I guess learn from it and adapt.
My 2 cts