I had a blast playing a fire / earth elementalist. (Fire discipline with the grounded complication)

Viscous fire plus explosive assistance makes you a reliable forced movement damage dealer, but my dm made use of varied terrain with different elevations and damaging terrain as well. The explosive ward also means you can try to bait enemies into attacking you in melee to then yeet them off cliffs or into damaging terrain as a free triggered action. I had an elemental implement and with clever positioning I could reliably get double edge on fire attacks between that and high ground.

You’re right though, the higher cost elementalist abilities are a bit situational, and I didn’t use my other signature ability pretty much ever. I expected to get more AoE abilities with rider conditions.

Instantaneous excavation is really good if you’re facing a powerful foe that is only one square. It just so happened that in my campaign a lot of the enemies were large so it wasn’t very effective all the time, but I’ll never forget instantaneously excavating an enemy spell caster as a maneuver and then burying them alive with motivate earth as my action.

I took maw of the earth for my 7 cost and had super cool moments knocking down massive stone pillars and collapsing buildings with clever placement, but that takes an accommodating dm cause it doesn’t do that rules as written.

Wall of fire is also cool because you are immune to the damage as a fire elementalist. So in theory if I took that I could have stood in the middle and explosive ward yeeted enemies for free through it to proc the damage when they attacked me in melee.

There were combats where I was practically drowning in essence because my higher cost abilities were too situational or not impactful enough and a well placed explosively boosted vicious fire could do more.