Lava wave’s main purpose is to get the experience rather than clear the wave. Any wave clear expert like Xul, can clear an entire wave as fast as he can travel down it, the minions might as well be made of paper. Ragnaross can do similar with the right build, without needing to pick Lava Wave.
The main value proposition of Lava Wave is that for the cost of a team fight ultimate, Ragnaross can remotely lane while participating in a team fight. There are some lesser cases where it brings value, such as against the zerg objective on braxis, or as a surprise attack when defending the core, but those will not be the main reason to take the ultimate.
So that he can lane while team fighting. If you pick Sulfuras Smash you have to pick either team fighting, or laneing since you cannot be in two places at the same time. Team fighting in this context is off-lane team fighting, such as over objectives, mercs or bosses. Sure Ragnaross can always lane later, but Lava Wave lets you lane any time it is off cooldown.