I dont see how this makes any fundamental change. Gauge will only accrue at X rate. So you still have a set number of times that it can he used. You are just replacing a timer with a gauge which is a fancy way of putting it on a timer. Theres no additional 'decision making' to a limit break than a cooldown.

'We get 1 lb3 in this fight. We must use it by X time otherwise we wont get a 2nd lb before the end of the fight so we must use it in this 1 minute window'.

My immunity is on X minute timer so I have to use it by Y time to still have it available for Z mechanic at the end.

Theres no difference. It's an invuln that's on a timer. If that is a prebuilt cooldown or a fight specific gauge is the same thing. You could get the exact same effect by having invulns recast start when you pull a boss if you dont want to have it available from the start. Pld hast to earn gauge for shelltron while other tanks can use theirs from the 1st moment. That doesnt make shelltron this amazing decisionmaking event. You just cant use it for 20 sec then it's the same as everyone else. It's the same thing.