The first equation in the paper is
$$\Delta P_t = \mu + \sum_{j=1}^J \lambda^j S^j_t + \sum_{p=1}^P \delta_p \Delta P_{t-p} + \varepsilon_t$$
Yes, it's a bit of a mouthful but clicking all of those symbols in equation editor is much slower once you know the syntax.
By contrast, LaTeX works the same everywhere -- to the extent that SymPy can derive an equation, output it as LaTeX, and it goes right into a paper or presentation without issues.
Also, like another poster has said, Word now recognises some LateX-like shorthand in equations so you don't have to click everywhere. Frex, adding a subscript is as simple as adding an underscore, the subscript and pressing space.
I don't know how to quickly add superscripts, or both sub- and super-scripts though :)
Not advocating for Word vs LaTeX, mind, but Word's actually pretty handy for writing a quick paper, like for a Master's thesis or putting together an early paper draft to get an idea of the outline etc. things that don't have stringent formatting requirements.
https://i.imgur.com/T9JxuH5.png
Fixing that for the summation terms and t-p subscript produces:
\Delta P_t = \mu + \sum_(j=1)^J \lambda^j S^j_t + \sum_(p=1)^P \delta_p \Delta P_(t-p) + \varepsilon_t
where Word doesn't realize that the terms after the sum should be the content of the sum. Word Help and the equation editor are thoroughly unhelpful in deducing the appropriate syntax of \sum.
What I eventually divined was that the syntax is state-based: If you type it out manually, it will put your cursor into the right position, and once you're done with _t you need to hit the right arrow key before typing the space and plus symbol. Copy-pasting the whole thing is different and does not work.
I mean, it's great that it supports an entry mode that's faster than digging through menus and clicking on symbols with your mouse. But it's still fundamentally different and, dare I say, inferior when the appearance of the equation isn't defined by the sequence of characters you enter (or copy-paste) but by the sequence of arrow keys and escape keys you hit while you type it in. This is exactly the problem with Office vs. LaTeX.