ok gamers, no stats here, i’m just throwing my experience into the mix, no bully and happy posting it (or else 
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The frequency I see about "Lava Wave too strong, whether here or somewhere else is getting a tole and honestly, I find it rather obnoxious.
Considering they always talk about how strong Lava Wave than Sulfuras Smash is in some situations (apples to oranges), let’s talk about why Lava Wave is actually not strong and is balanced to depending on how you handle your macro and soaking game.
So what is Lava Wave? Big button, select lane, unleash your 12 year old gamer rage on that ant hill next to your neighbors wall screeching "DIE INSECT"? That one, pretty much deletes all minions, heavily damages anything else but also soaks pretty much everything with no worries? Yes that’s the one.
In concept Lava Wave gives you the VIP ticket pass of soaking leisure by telling you to not soak for picking a bruiser for the offlane with actually respectable wave clear and a 4 second minion wave clearing by pressing 2 Qs (when the lvl 1 quest is complete) and some AA in 4 seconds. In one way or form, Lava Wave shut down or heavily weakens a macro pressure that would end up taking a fort/keep by not taking one.
But like any heroic, what’s the cost?
Let’s take a look at the fire lord himself, his entire kit is composed of AoE, his Q, W, E and R, aka his entire kit has an AoE attached to it, however unlike the majority of bruisers he struggles in being purely one of an absolutely GANGABLE of a hero, unable to counteract if he’s engaged, this soft patches itself in level 4 if you are using Slow Burn (40% slow for 2 seconds) and then utilize Blast Wave as an engagement/disengagement than anything else, you can see that Ragnaros doesn’t relaly have a lot of play making potential due the lack of CC, Xul point and click root, Dehaka’s tounge, Sonya, and Imperius (when the stars aligned) can achieve that, of course they have their own pure weakness but that’s a discussion for a different thread.
So you end up with, a rather an abundance of wave clear attached to a hero but no form of useful CC unless talented to. If we look at this, Ragnaros doesn’t really bring much to the team fight from a baseline perspective (unless W build is involved) however this is where Sulfuras Smash becomes prominently promsing, as much as people say it is a difficult heroic to cast due its telegraphed and respectable delay, it’s certainly one of a strong tide breaker in pretty much pushing the fight in your favor and for once give Ragnaros a rather strong heroic, this makes Slow Burn also really useful in landing it, a 40% slow can give it to you guarantee in majority of cases and allows turning the fight in your table, not only for you, but your tanks and teammates.
What makes lava wave is seen as OP is the result of it denying lanes but if we look at the fire lord you sacrifice a rather strong engagement tool, even more if you are purely talenting for Q or unironically E build (
), I don’t think a lot of know about this but there’s actually some timing involved for Lava Wave in which it becomes even better to cast and worse to in other cases to be casted, see this article for more info.
Otherwise, Lava Wave like was said, its the VIP pass of soaking, this makes it promising in the lower leagues but not really in the higher leagues, the better you soak the better you will do “as Ragnaros”/“against Ragnaros”, the less you have to worry about a global pressuring ult, Lava Wave is the epitome of forgiveness because it does something that strangely a lot of people struggle in doing which is soaking a lane and then rotate to the next, do some deniability for the laner so they take the longer route and go back and forth.
Oh and for the record, don’t use lava wave on the zergs in Braxis Holdout, that’s cringe, it deals so little damage (7%~), go as the big boy and shut down most of the wave before it reaches you.
So in conclusion? Lava Wave is surprisingly balanced, pick Smash more and most importantly:
Soak and play offlane better lol.