Rumble's only op if you don't have any MR.
His flamespitter CD already has been nerfed before: They added another second to it as well as reduced the AP ratios. Also, Rumble has flamespitter off as often as he has it on due to CD. He also can't leave it constantly on or else he'll overheat, and Junkyard Titan no longer procs Rylai's.
His 1 second speed boost is neglible for escaping. Rumble shines at hunting people down, but escaping is only mediocre.
Tiapoa sums it up well:
A good rumble player needs to capitalize on the early game and get in as much as possible because the late is going to suffer.
That is true because in the early game, fewer people build their tanky items.
Basically, Rumble will eat squishies who don't build MR.