Holy ish! Neovim has progressed so much. Exciting.
I'm actually using it currently and I really enjoy the built-in terminal. It makes my workflow much better.
Be aware that some plugins might have some bugs. I also got segfaults when using multiple terminals (although that's quite rare).
Interesting. I tried out the built-in terminal and found that I couldn't navigate around panes as quickly as if I just used tmux instead. Are you using the default mappings, or have you made them easier to use somehow?
The current defaults discussion is at https://github.com/neovim/neovim/issues/2676.
Actually, one of the main goals of Neovim[1] is to provide better defaults[2], such has syntax highlighting.
[1] https://neovim.io/
[2] https://github.com/neovim/neovim/issues/276