Lava Bombs require Mythril Topaz, which are fortunately much less rare than Mythril Lodes. I've found them useful when I need to save up certain Tier 1 ingredients for whatever reason, and of course having two different elements' special effects is always welcome.
Their main drawback is probably that the dual element is something of a double edged sword- it can give them more chances to hit weaknesses (including Banes if you get the chance to throw a few of those down), but they can also get blocked by two different resistances and one immunity knocks their damage to zero.