Quoted from: Halliwell, George R. "Evaluation of vertical coordinate and vertical mixing algorithms in the HYbrid-Coordinate Ocean Model (HYCOM)." Ocean Modelling 7, no. 3-4 (2004): 285-322.  https://doi.org/10.1016/j.ocemod.2003.10.002 

Advantages of isopycnic-coordinate models such as MICOM include: (1) the absence of numerically induced diapycnal mixing and (2) the ability to efficiently resolve baroclinic structure with relatively few vertical coordinates. Disadvantages include: (1) providing inadequate vertical resolution in regions with weak stratification, including the surface mixed layer; and (2) being suboptimum in the coastal ocean and shallow seas where topographic changes are large and bottom boundary layer dynamics is usually important. All MICOM isopycnic layers with reference densities smaller than the mixed layer density collapse to zero thickness at the mixed layer base (MLB). Only slab mixed layer models can be used, and MICOM is equipped with a Kraus–Turner model (Turner and Kraus, 1967; Niiler and Kraus, 1977) that employs the modified TKE parameterization of Gaspar (1988). The HYCOM hybrid coordinate algorithm was developed to overcome these MICOM limitations by placing different vertical coordinates types in those regions where they are quasi-optimum, and thus permitting the use of more sophisticated turbulence closures.