Vundle is really neat and so is vim. But some plugins requires external binaries or services to function and that's when a vim packager stops being helpful and you have to take extra steps. Sometimes it's just an apt-get install something or curling a bin to ~/.bin but sometimes it needs more tweaking and fiddling. In my opinion that makes your vim installation more fragile and less portable.

Maybe we'll soon run vim in docker.

Vundle does install YCM, but you need to execute some more steps (compile the ycmd server), which vundle doesn't do, and YCM doesn't work without the daemon compiled.