Finally I voted the poll: No issue, everything went smoothly 
The first thing that I’ve done after the updates has been to install libxfce4ui-nocsd (I had already installed Gtk3-classic from AUR - now again available in the official repo); I also had to set DialogsUseHeader to false in
Settings Editor > xsettings > Gtk, which is easily doable by executing xfconf-query -c xsettings -p /Gtk/DialogsUseHeader -s false in a Terminal window.
I didn’t faced issues with lost settings in Whisker Menu; I didn’t lost the battery icon(s) - my icon theme has the “level” icons for battery (battery-level-*-symbolic.svg).
Some minor annoyance was the fact that some other icons, like eg the “Show Desktop” have changed the name scheme: previously was named show-desktop.svg and was available in the icon theme folder; now is take from /usr/share/hicolor/32x32/apps/ and is named org.xfce.panel.showdesktop.svg; so I copied the previous icon from icon theme folder, as ~/.icons/hicolor/32x32/apps/org.xfce.panel.showdesktop.svg and is like before
And as I’ve said above, the battery icon in the panel is big/huge.
Obviously I also run PACDIFF and the only pacnew was the one of dnscrypt-proxy, which for me has only introduced new phrases in the config file.
Xfce 4.16 seems to acting better than before: is blatantly faster/smooth; the panels are improved (before, when set to auto-hide, they had the tendency to not hide/unhide properly).
HINT
Now, the panels, when set to autohide, they have a fade out animation, which, by default, imho, is too slow.
Is possible speed it up by adding the following code to ~/.config/gtk-3.0/gtk.css:
.xfce4-panel {
-XfcePanelWindow-popup-delay: 100;
-XfcePanelWindow-popdown-delay: 10;
}
Or by further lowering these values.
(personally I prefer -XfcePanelWindow-popup-delay: 5 and -XfcePanelWindow-popdown-delay: 2;)