I read that it is impossible to have a full cadence using all three notes of the final triad in a 4 part counterpoint piece. Here is the exact bit of info:
"we can’t easily arrive at a full triad in the final measure. The bass and alto (or whichever voice has the CF) must both go to ^1, the tenor is free to go to ^3 or ^5, but the soprano usually has ^7 in the penultimate V chord, so the leading tone must resolve to ^1. It is very normal to end on an incomplete I chord for these reasons."
Can someone please clarify if this is true and why? Most music I have listened to always has a full triad at the end of a cadence. And when I studied harmony it has always been a V > I progression, meaning that the whole I chord is used. Where am I getting confused?