I am not sure how worth an exploding Mega is (I am speaking of Glalie). If I remember correctly, Jolly Explosion does kill 4 HP/ 252 Def Eviolith Chansey, but what does it do otherwise?
Is the offensive pressure alone really worth for a Spike-Suicide Lead when it is costing you a mega slot?
Just think about that for a second, if it can actually take hits with 80/80/80 and ice typing and being SR weak.
Basicly what I am trying to say, for that insanely powerful Explosion, nothing is immun to, you are wasting a Mega Slot that you can use for other, more versitile Pokemon, and with the huge selection of Megas we are getting, I am sure you can find at the very least 5 that could fullfill that expectation.
Unlike with other Exploders in the past, while you don't have to worry about Ghost Types, you want to save your Explosion for a centain threat in a team. Like you want to kill a Chansey but your opponent switches a weakened Landorus, while it might be a threat, it could have been something you might have handled with something else rather easiely.