I'm not sure why a feather binary isn't available yet, but the issue with stringi is a known, frustratingly persistent problem:
Due to stringr's dependence on stringi, I've seen this causing a lot of pain for people trying to install a range of packages for R 3.5 on Windows. Many (most??) such users do not have RTools installed, so by default install.packages() will just keep installing the very old version of stringi. People who don't have RTools installed are also very likely to have trouble debugging this obscure web of dependencies! So thanks very much for leaving this note here
