I've been in a few games where the Elder Dragon spawns and is killed. My experience has been that the buff feels irrelevant. I understand that it's going to be weak for a team that didn't prioritize the elemental dragons in the early game, but even in games where one team gets 5 Elemental Dragon buffs, it seems to have no effect on the game.
The main issue is probably the comically short duration on the buff. Stalling out the game for 90 seconds is not difficult at all - especially when the enemy team had just committed a lot of time to slaying a super tanky boss and so now probably have 2-3 lanes that are pushing against them.
I can understand wanting the buff to be a shorter duration than the other late game boss option in order to differentiate it, but the present duration is almost always too short to mean anything at all.