Well technically the wraithknight is undead, and the riptide and dreadknight both have living pilots inside. The wraithlord was always a monstrous creature.
Well, isn't a dreadnought also "undead" in exactly the same way as a wraithnight (or a Wraithlord)? And, having a living pilot inside sure makes it sound more like a vehicle. After all, Sentinels, Vendettas, Rhinos, and Leman Russes all have living pilots inside as well.
Actually I vaguely remember from high school (before i stopped playing for 15 years) that what we call a "Wraithlord" now used to be called an "Eldar Dreadnought"?
It's kind of funny... they still count a Soul Grinder as a walker, and Soul Grinder actually IS half alive. It's more like a monstrous creature than any of these constructs IMHO...