I have a 4-wheeled wheelbarrow/cart/whatever. I think Scotts made it, but I could be wrong (it's green with orange lettering); I got it at some big-box years ago. It's wildly easier to move than my regular wheelbarrow, precisely because you expend no effort balancing it, and it's equally easy to use pulling as pushing (even with kinda crappy small, hard wheels). It is definitely not as maneuverable as the traditional, but it's easier on (most, not all) 'uneven terrain' (again, I don't have to balance it) and it doesn't have any 'steering' other than pushing the handles so it points the direction I want to go (I do know there are more elaborate versions that have a steerable set of wheels; I got the cheap version).

I guess tl;dr: I dunno what these guys are going on about and wonder if they've ever actually tried a 4 wheel cart.