I don't think they do too well with too much nitrogen. Also does poorly in area that doesn't drain well / gets too much water.
Dramatically better performance when inoculated with rhizobium for legumes before sowing. I don't know if it helps to work some into the soil after they are growing, though it seems like it should.
I sowed uninoculated vetch last fall for cover crop because I was pretty sure my garden has grown enough peas, beans, clovers, vetch, etc. in the past. In the course of weeding, I've pulled some up, roots and all, and am finding gratifyingly numerous root nodules.