Hi, neovim core member here.
Which UI plugins do you need? We've done a lot of work leading up to 0.5 in improving the built-in handlers, so you shouldn't need a UI plugin. I think the main plugins people install (in addition to lspconfig):
* nvim-compe (autocompletion)
* vim-vsnip (snippets)
* lsp_signature.nvim (automatically pop up signature window, note signature_help is built-into core, just manually triggered)
Some people use lspsaga.nvim, but borders are already merged into the core handlers (and our floating windows now provide better markdown styling than lspsaga), so the main utility of lspsaga is in the different way of interacting with code actions, which I personally do not prefer.
Anyways, please open an issue or PR if you have concrete suggestions (or start a discussion on our discourse)! I'm personally very invested in improving the UX around the built-in language server client, and it has really improved leading up to the 0.5 release.