I've been an MFE since BF LH and I think it has its advantages and disadvantages that balances it out.
In GVG, I see no need for MFE's to be nerfed. As gvg's pretty much whoever hits first (with a crit or a stun) wins.
In PVP:
MFE with the mana shield, GS has no chance but without the mana shield, GS has the advantage.
MFE is obviously disadvantaged against gunners due to range and no disabling capability. (Ex: I can only win against SanD if my moonflame crits high enough)
MFE vs SG is equal in my opinion. Shooting Star vs Onslaught.
In PvE, and for example MFE vs Ossus, I have a problem with that.
I can pretty much tank 3 dragons but I cannot damage any of them.
While MFE puts out huge amounts of damage, we're restricted by a CAPPED casting time and cooldowns. No matter how high my cast speed is shooting star is capped at 2s and fireball at 0.4s IIRC and they have cooldown that prevent them to be spammed *. The only alternative is using continuous skills like lightning & AoE which have absurd per-second rates.
*And then there's the "glitch". Due to latency or wtv reason, if you spam Fireball/Lightning/Solar Storm/Arctic blast, the skills can be multicasted (ignoring cooldown & casting time) occasionally. And this is probably the only reason why I've been able to kill in PVE at an acceptable rate. This bugs certain pet combinations tho which reduces damage output.