I am currently trying to make fur/ grass for a fantasy character using Eevee as the renderer (because Cycles really does not provide that much of a visual upgrade in this specific case). The problem I am running into is that the hairs seem to merge together into an ill-defined blob. I thought this was because of resolution, but every other detail is so much clearer. I have tried fixing the issue by decreasing the filter size in the render options, and nothing in the hair options seemed to fix this issue. Everything looks good in the preview, but the render itself fails. I also tried messing with near clip in the camera, which also fixed nothing. Here is the link to the blend file: https://www.dropbox.com/s/jtf1esgqvd8o1ll/Dryad_Hair.blend?dl=0
The viewport and the render being different likely has to do with the children settings. Change the number of children to be the same for render and viewport. If this doesn't work, check out the render and viewport display panels, then finally the Hair panel in the Render tab (as opposed to the PArticles tab).