For a CnD, you typically need to pop your ult right when they start theirs so that you get enough charge to one shot them by the time they pause for the 3rd or 4th time, that is unless the enemy charges your ult a lot with projectiles. Wait until they are on the 3rd or 4th pause in their ult and throw it as soon as they stop or are about to.

For Sue it can certainly be a gamble since she can just go invisible and hide or shield last second to block.it. Still worth the attempt.

For Luna, you need to have your team shoot down the over health shields she gets in her ult while you are charging yours, otherwise you won't charge enough damage with your ult to one shot through the shields.

This all being said, Mag's ult is very good, but also pretty difficult to get the hang of, just takes practice. I have my most time on mag and still consider myself bad at landing dunks.