[Otaku]

For those using Insubstantiality as Invincibility, remember that means the source of damage is still there/passing through you. Under most circumstances, I don't think that can be handled as fluff. Where it does work is something like Superman getting caught in a big bomb blast and coming out fine. Where it doesn't work is Supes shielding innocent civillians, allies, etc. Then there's the bookkeeping (though maybe Cosmic handles most of that) remembering all the stuff about "true" Insubstantiality that applies to Invincibility-As-Insubstantiality.

Maybe use IaI as an Alternate Ability to the appropriate amount of DR/IT?

I know I'm switching (sub)topics, but when it comes to using Insubstantiality offensively, as in, to directly harm someone, I recommend just having the character buy the appropriate form of Innate Attack (or similar Advantage), and the fluff is that it happens via their "phasing".