Haven't used plank. Have used cairo-dock, but on debian-jessie, and it was causing spurious logouts in response to some window resizing operations on this xps13. The bottom "panel" looks like cairo-dock, so i'm being a leery old fogie.
One of the best things about Ubuntu-MATE is that it uses the sameold look and feel as the gnome-2 panel manager, which is the one that was on the first Ubuntu version i tried, just before Unity... arrived. I find it to be a kind of haven-of-safety thing to know there's a distro that works like Ubuntu used to work before they... fixed it. It's the customization. I tried XFCE and LXDE on Debian and could never get them set up as nicely as the ol' standby setup. The cairo-dock has an interesting customization interface, and maybe plank is different, but i like the simplicity of the existing default.
It's not that i'm against change (any more than the next guy) but knowing that if push comes to shove i can install Ubuntu-MATE on whatever hardware is available and not have to dig around through unfamiliar stuff to find out how to get the "old default" back is comforting... maybe i have trauma issues from having to unhook Unity from 11.10 long ago.
I'm all for adding new coolness for folks to choose from, but when i enter a room i expect the floor to be underfoot each time i walk through the door.