In other words, once you learn the basics in TeXmacs, you can enter all kinds of content in the same way — more so in fact than with any other WYSIWYG competitor. Is this true?
Is TeXmacs the best WYSIWYG word processor in terms of entering all kinds of content in the same way?
I do not know if the entering of content is as uniform as possible; the mechanism of cycling through symbols using tabs is quite good IMO—and that is uniform; I do not have an opinion on the composition of symbols using multiple keys as I use them very little (I have in mind only the arrows made e.g. with =>).
Said this—the key to TeXmacs is that it is WYSIWYG and structured (very important 
)