I use ConqueTerm, inside vim, a plugin that supplies a terminal inside Vim. If you put the emulator inside the editor, you can take advantage of vim's extremely capable text manipulation. If I tmux then vim, I have to find some way to get the text out of tmux, and then back into vim.
Living inside the terminal, multiplexing is useful even when not using Vim and I think Tmux is a better multiplexer than Vim. I would need to use Tmux anyway for it's sessions.
I already use Tmux inside a tiling window manager and generally try to avoid manually managing panes inside Vim, mostly only tabs. There is a finite amount of hierarchical panes and tabs you can work with intuitively, before you wonder why they won't accept each others keyboard shortcuts, especially when you use the same color scheme in Vim, Tmux and Awesome.
> If I tmux then vim, I have to find some way to get the text out of tmux, and then back into vim.
That's actually a valid concern. I use piping for that, but it's not ideal at all. Maybe I will try it out.
> There is a finite amount of hierarchical panes and tabs you can work with intuitively, before you wonder why they won't accept each others keyboard shortcuts
Exactly the reason I want to manage terminals in vim. I already heavily use splitting in vim, and find it much faster and simpler to use than screen or tmux. In addition, I typically want integration between those splits, such as displaying the quickfix list in one, or showing 2-3 files in vimdiff, or yanking and pasting between files. Given that, I'd like to just open one more split in vim and have a terminal in it.
The original motivating use case for me: a vertical split, editing a manpage on one side, and showing a continuously updated render of the manpage on the other (using watch).
I was playing devils advocate a bit in the last post ;-) My workflow involves a mix of the two methods. If I know I'm going to be doing text manipulation, I'll use vim+ConqueTerm, but more normally I just let iTerm2's splitting handle it. (I actually prefer it over tmux/screen, but when you need multiple things on a remote, I can see why people prefer it.)
Depends on how you implement it. neovim's implementation uses libvterm, which seems preferable to a from-scratch terminal implementation. Implementing a fully capable terminal emulator from scratch definitely isn't a weekend project.
neovim's approach links libvterm in via C. You could potentially do so via python and ctypes instead, but then you have to count on having a vim compiled with Python support.