A big disadvantage with TUI based email clients is it can only work with English or latin script based languages. Complex scripts(https://en.wikipedia.org/wiki/Complex_text_layout) does not render properly in terminals.
Recent versions of Emacs support a modern text-shaping engine, Harfbuzz, when run in GUI mode, which is another option for emacs-based mail clients. I guess it's maybe not technically a TUI in this case, but emacs mail clients are still very TUI-like, even when run in non-terminal emacs.