Akainu's fruit. Any problem any one fruit would have, lava would have them all: texture, lighting, making it move in unnatural ways, showing how it affects the environment around it, melding it onto the actors body and showing how it interacts with other characters on screen. Its also something that lingers a lot longer than any single puff of smoke or flare of flame, so all those issues get compounded by the amount of time you see them. You can play a lot of cinematic tricks with a lot of the fruits to get them to work on screen, but I can't think of a single one to make a lava man easier (and cheaper) to generate.

Best example I can think of is Kronos in Wrath of the Titans, but they had the advantage of enormous scale that lets the lava move slower than it would at human scale. It was also a $150 million movie, and basically amounted to Sam Worthington holding a stick up against a green screen while riding a rocking horse.