Does the 40' high cylinder imply that this couldn't be cast, say, in an underground room with a 10' ceiling height?

It does not

The way I'm reading it, there are two ways you could interpret it. 

A. (RaW, based off mechanical description) The cylinder's origin will start at a point you can see and go up or down exactly 40 feet, "clipping" (like in a video game) through any floors/ceilings if need be.

B. (RaI, based off visual description) The spell creates a circle of hail (ice) at the origin of the cylinder that falls down, up to 40 feet, until they hit something. It could also be interpreted that not all the hail starts at the top of the cylinder, so that the spell can still effect those under something.

Casting word is:

Chrystalus Cyclonium

So this is a 4th level spell that does 23 cold/bludgeoning damage and fireball is a 3rd level spell that does 28 fire damage. Is the split damage type and 1rd of difficult terrain really worth it?

Ice storm used to not have a saving throw and that was its main appeal.

Magical Bludgeoning and Magical Cold isn’t very commonly resisted damage compared to fire. I have a feeling that this, combined with the overall bigger area, and difficult Terrain is what lead to this spell being as it is.

Per their own definition in the DMG, Fireball should be a 5th lvl spell

So maybe don't compare everything to Fireball, Fireball wins

Thanks dmforl1fe, your response was insightful and helpful. :)

You're very welcome!

Quote from dmforl1fe >>

Magical Bludgeoning and Magical Cold isn’t very commonly resisted damage compared to fire. I have a feeling that this, combined with the overall bigger area, and difficult Terrain is what lead to this spell being as it is.

 I believe there are 2 creatures that are passively resistant to magical bludgeoning damage, the treant and wood woad, making it an excellent damage type. Cold, however, I think is the most resisted, with fire coming in second and having many more things immune to it.

Thankfully, upcasting this grants the better-scaling bludgeoning damage, and Elemental Adept helps cover against those pesky cold resistances. The increased spell level versus fireball is likely due to the difficult terrain it creates, making it hard for your targets to reposition themselves for a follow-up fireball. It also doesn't destroy all the combustible treasure in the room, unlike its lower-leveled cousin.

What die do you use to cast this

Quote from wanderinghistorian >>

So this is a 4th level spell that does 23 cold/bludgeoning damage and fireball is a 3rd level spell that does 28 fire damage. Is the split damage type and 1rd of difficult terrain really worth it?

Ice storm used to not have a saving throw and that was its main appeal.

 Less Damage but much larger AoE

Huge range  – 300ft. 
Big area of effect – 20 ft. radius and 40 ft. high so you could target flying creatures as well 
Difficult terrain – not only on the ground but also in the air, which means you can block flyers, deal a lot of dmg and stay away from them. 

Mixed dmg resistance also help in many cases. Overall, I wouldn't say it's much better than (bread and butter) Fireball but it give a lot of utility/defensive aspects as well. 

Quote from RWHellborn669 >>

Quote from wanderinghistorian >>

So this is a 4th level spell that does 23 cold/bludgeoning damage and fireball is a 3rd level spell that does 28 fire damage. Is the split damage type and 1rd of difficult terrain really worth it?

Ice storm used to not have a saving throw and that was its main appeal.

 Less Damage but much larger AoE

It's not really a much larger AOE.  Both have a 20 foot radius.  The 40ft cylinder is bigger but it's a niche situation that would require you to be able to hit both land and air targets and the fireball could do both as well just with half the area of the column within its sphere, making the situation even more niche.  

I can’t believe so many people are complaining about this spell! My Sorlock used this right up to 12th level before I got her blowed up by a beholder. My DM absolutely HATED this spell!! And it didn’t die with poor Samara either, it became a group favorite!

A super long range AOE that also slows the target (even flyers!) on their next turn means a lot of kiting potential. A lot of DMs don't include wide-scale outdoor encounters (they should) but on a big battlefield, this is an impressive spell.

Quote from Tyranthraxes >>

Casting word is:

Chrystalus Cyclonium

 

 

Beyond 20

 

 

 

 

 

 No it isn't 🛑

Quote from StopDoingThis >>

Quote from Tyranthraxes >>

Casting word is:

Chrystalus Cyclonium

 

 

Beyond 20

 

 

 

 

 

 No it isn't 🛑

They've been posting these self serving, self important "Casting Words" for years. Best to ignore them.