A friend of mine also stole baron with Rumble's ult without a full AP Build.
Let's not forget that they had a fed Vayne on their team, meaning they could just destroy Baron.
Isn't it easier to steal Baron with old Rumble Ult that had burst damage, instead of getting lucky with small ticks of DoT which fall between Vayne's AA's?
Also, did some math on my earlier statement that Flamespitter > Equaliser at Lv 6.
Flamespitter Rank is + 70 damage, 6 sec C/D. + 97.5 damage in Danger Zone.
Because Rumble cannot keep a target in The Equaliser, and it can be left in 1 second regardless of how slowed you are, I gave the benefit that the foe doesn't have a skill that just lets them jump out instantly therefor taking only one tick of damage, and gave Rumble's Ult 1 second. That's 130 + 30% AP, on a 105 second cooldown.
So... 60 [+30% AP at Lv 6 when Rumble has almost none]/32.5 more damage... on a 99 second longer cooldown.
I think I know which one I should be taking at Lv 6, considering most facebashing contests toplane take longer than 6 seconds, and Flamespitter is also your main farming and harass tool.
To get more out of Equaliser in a toplane scenario early-game, without a gank holding them down, you'd have to assume your foe is stupid and stands in your Ultimate. Usually in these sorts of things you assume the highest skill level possible. On the other hand Flamespitter turns with Rumble, and follows the target as Rumble chases them, so running from that is futile, as you're just taking free damage.