As I answered in the issue [1] the problem is likely caused by the task timeout. Maybe vim-plug should delete the repo directory if timeout occurs during the installation. A workaround at the moment is to increase `g:plug_timeout` so that it never times out.
FYI, vim-plug also supports lazy-loading and the interface is pretty simple and concise [1].
NeoBundle obviously has more features, most notably supporting VCS' other than git, while vim-plug tries to be simpler and easier to setup and use (being a single file, no boilerplate code [2]). So they have different perspectives and I wouldn't argue that one is better than the other. It really depends on personal tastes.
The most obvious benefit I believe is its ability to update plugins in parallel. A GIF [1] shows that it managed to update 40 plugins in mere 4 seconds. The other notable point is that it allows you to load plugins on demand, which helps reducing the startup time greatly [2] if you happened to use many number of plugins. It also provides some other nice features that pathogen or vundle do not have while still being easy to use.
Not perfectly fine if you use MySQL. Adding an index to a table on MySQL locks the entire table, which can introduce hours of downtime if the size of the table is large.
