The usual argument against it is that if behavior of matter is not fully determined by its state then it has to be determined by something else, ergo dualism. This begs the question however, unless we insert determinism into the definition of matter its behavior can be causally gapped, there is no sufficient cause, no poltergeist, no "else".
What I think some libertarians about free will have in mind is something like this. We have basic level objects, fields and particles, we have weakly emergent high level objects, let me call them neutrally "collective states" (think of global patterns on a lattice grid). Basic level evolution is probabilistic, and there is a feedback channel that lets collective states influence distribution of outcomes in basic events, that is alter their probabilities. The influence is causal but lawless, not subject even to probabilistic laws, or partially lawless. One might object that there is no plausible mechanism for that. But we do not have a "mechanism" for matter "generating" gravity either, just an equation relating metric tensor to stress-energy tensor. Presumably the strength of the channel depends on complexity of emergents. With our present experimental capabilities, that require setup isolation, any such effects will be undetectable. But they may become detectable eventually as lawless deviations from pure QM predictions, etc. The scheme might require non-locality (although with some creativity one might be able to eliminate that), but it does not seem to require poltergeist.
This is only meant to give an idea, details are malleable. Think of the basic as physical, and collective as mental. One aspect usually assumed about reductionism will be missing: because of the feedback channel there will be no reduction of collective evolution to basic evolution, the basic states alone do not constrain it fully. But this concerns epistemological reduction only, ontologically there appears to be nothing but matter.
Would a materialist/physicalist accept this kind of reduction? The idea, as Davidson put it, is that mental events are physical events, but no laws relate mental to physical. He was a compatibilist though. Are there ways for a materialist to accept libertarian free will? If not what will be the arguments against (but please do not make them dependent on specifics of the above scheme)?