Actually, one big (huge!) feature of NeoVim and Vim 8 is actually meant for plug-in writers, which is the ability to do asynchronous communication with external jobs.
This is especially useful for plug-ins that communicate with external commands, doing tasks such as completion (YouCompleteMe, Deoplete, plug-ins that communicate with a language server using the LSP protocol) or linting (ALE, etc.)
He also suggested I used it without any plugins, but is the autocomplete function on NeoVim better than normal Vim's?
Not really. Vim's autocompletion features have been around and haven't been significantly changed in NeoVim or Vim 8.
I imagine your friend's recommendation to use it without plug-ins is so that you can experience the full power of native Vim autocompletion and learn to use its advanced features.
Personally, I don't really use a plug-in to do autocompletion, but I do generate
ctags for most projects I work on and I use completion from tags, which I find does quite a good job for my needs.
I actually do use a plug-in to generate and keep the
ctags file up to date, though.
In the end, it's up to you to decide how Vim works best for you. If you're happy with your current plug-ins, by all means keep using them in NeoVim! If you would like to see more of what it can do out-of-the-box, then disable the plug-ins to "force" you to learn the native way. And don't forget the two are not mutually exclusive, you can still use native completion even if you're also using a plug-in!