Undispellable Magic Guard?

Such a non issue. Just recast when dispelled. You'd never die if you were paying attention.

Now damage reflect bypassing magic guard is pure BS.

Casting Meteor, Booster, Poison Mist or any other long casting skill (or long macros) gives time enough to get dispelled and killed before you can recast MG.
I know it's happened many times to me at Pap before I had enough HP/MP to stand at the top (pre-BB).

Of course it's easy to stay alive when only using Heal or other fast skills, being close the US and paying close attention to your buffs. Try doing that when you have to keep a lot of DoTs active, pay attention to 2 cooldowns all while juggling buffs, taking care of both HP/MP so you don't die if you get dispelled and still managing to survive the longer delay for living outside the USA.

Then there's of course the classical stun+dispel or seduce+dispel that's much more deadly to mages than any other class.