Same experience. It was smooth 2 years ago when I tried it so I thought it be more polished now... Color me frustrated
As recommended by michaelsbradley below, I installed https://github.com/nim-lang/langserver. I'm using coc.nvim (https://github.com/neoclide/coc.nvim) so I followed the instructions here from nim langserver https://github.com/nim-lang/langserver#vimneovim and seems to be working well!