The 2017 detection of the in-spiral and merger of two neutron stars was a landmark discovery in astrophysics. Through a wealth of multi-messenger data, we now know that the merger of these ultracompact stellar remnants is a central engine of short gamma ray bursts and a site of r-process nucleosynthesis, where the heaviest elements in our universe are formed. The radioactive decay of unstable heavy elements produced in such mergers powers an optical and infra-red transient: The kilonova. One key driver of nucleosynthesis and resultant electromagnetic afterglow is wind driven by an accretion disk formed around the compact remnant. Neutrino transport plays a key role in setting the electron fraction in this outflow, thus controlling nucleosynthesis. Collapsars are black hole accretion disks formed after the core of a massive, rapidly rotating star collapses to a black hole. These dramatic systems rely on much the same physics and modeling as post-merger disks, and can also be a key driver of r-processes nucleosynthesis. I present recent progress in modeling these enigmatic systems, with an emphasis on both the impact and techniques of detailed Monte Carlo neutrino transport.