Hydrogen also should enable more hydrocarbons to be produced from biomass, by hydrodeoxygenation: (CH2O)n + n H2 --> (CH2)n + n H2O.

IIRC traditional Fischer-Tropsch synthesis of liquid hydrocarbons from coal or biomass is about 30-40% efficient. Partly because the C:H ratio of the source material is not the same as the end product (the excess goes up the smokestack), but also because part of the energy embodied in the source is needed to run the reaction itself.

Adding hydrogen can fix both of these problems, enabling efficiency up to 150% (compared to the energy in the source material, not including the hydrogen). This is kind of a big deal, since for large-scale carbon-neutral production of synthetic hydrocarbons the bottleneck is the source of carbon. Hydrogen can be produced (nearly) carbon-free in unlimited quantities, but barring a breakthrough in DAC technology where is your sustainable source of carbon? And no, we don't want to replace pristine rain forests with biomass plantations.

What's wrong with biomass plantations as a source of renewable carbon, assuming you don't build them on rainforests?

From a biodiversity point of view. Everything.

The generic problem with biomass is the low overall efficiency of photosynthesis. It doesn't work well when it gets too cold or hot, and can use huge amounts of water.