In practice multiple applications get confused, and some don't support -geometry or equivalent.
One specific case I dealt with was Chrome/Chromium that provided all sorts of annoyances until we dropped in a minimal WM (back then it was awesomewm, I didn't know about cage or it didn't exist yet)
In ye olden times, when most X11 applications understood "-geometry" and didn't try to do too much in absence of WM, things worked.
Chromium and possibly others (Chromium is just what I have personally dealt with) was confused about what did it mean to be fullscreen, and various other ideas regarding placing itself on screen.
I did success in making it work for a time, but it was problematic.
