Yeah, that's true. Tho for large pastes, I generally only do that in a locally running GUI emacs. For remote, I'll assemble the file and then scp.

Even without tmux, this can be an issue - a long time ago I was using the X windows "drag the selection" middle mouse button action and accidentally let go of the button while the cursor was over an emacs text window - I sent my first email to some big listserv I subscribed to consisting of weird fragments from the log file I was trying to paste.a