the logic of industrial scale ag can make anything bad.

I used to work in a farm where we grew peanuts, but only like maybe 100 row feet or so, and rotated with dozens of others crops. it was a very sandy, coastal soil so we could harvest by hand with a digging fork, though like most annuals, planting involved tillage. the impact was reduced by how sandy it was and we weren't assholes, so we kept passes to a minimum.

I've seen peanuts grown at a slightly larger scale in southern Japan, think like 50' thick strips along the borders of fields stretching for miles. also southern/coastal so still sandy.

I've never really seen big monocrop peanut fields like I'm sure exist in the southeastern US, but I'm sure it's rough.

the general benefit of legumes grown for crop is less that they leave fixed nitrogen behind and more that they fix their own instead of requiring it be brought to them. that's a big deal in terms of energy expenditure and global warming potential of the production system.