The one disadvantage of that is you cannot easily see all available functions at once. It loses discoverability. Autocomplete on the input prompt will help rectify that somewhat, but only somewhat.

The discoverability argument doesn't make a lot of sense for a calculator app.

One might want to list the content of a directory without knowing that the magic word is "ls", but there is a 0% chance that someone wants to compute the cosine of a number but can't figure out to type "cos".

But is it cos x, cos(x), cos[x], math.cos(x), or \cos x? Calculators are usually simple enough that all the ops can be buttons. So why not do it? There's always alternatives like Python, Mathematica, Maxima, Fricas, etc. which don't really bother with the buttons, because if they did, it'd look like a Chinese typewriter.

Additionally it also hinders those who do not natively speak English. Sqrt or square root and pow aren't terms that commonly used in calculator for other languages, at least my country.

And I guess sqrt shortform isn't that common used either.

Able to see symbols can clear those ambiguity

I don't mind the buttons for special functions, but for numbers and operators? Yeesh that's redundant. I can type 0 through 9, + and - without needing discoverability for sure. At minimum I assume the calculator does arithmetic. Buttons for special features tell me it does other things too.