So, while now I'm fully aware that the factions is fully balanced and every units have their uses, I do notice that some units are very rarely encountered in Multiplayer games. Yeah. perhaps they could be useful in the hands of skilled players or when their usage is unexpected in and on the act itself. But still, you very rarely bother with them...
Lemme start first:
For Both Sides :
* Supply truck, while they're definitely useful when your allies are broke and need some help for his economy, they're pretty much useless otherwise. Sometimes, I wonder what if we can use typical RTS resource sharing method by direct credit transfer instead? While arguments about the enemy should be able to intercept the money in their journey are good, you should remember that most of the time (if build over ally con yard is on), more often than not, you will notice about your allies building near your MCV, ESPECIALLY at the expansion sites (and you are often do that as well), making the transfer is a bit odd since your war factory is very near with his refinery, so why not just direct transfer?
* Minelayer : Theoritically useful, but their 3 mines expanded quickly before you must re-fill them. Unless the map was littered with chokepoint they're practically useless because your enemy just need to bombard the ground with arty / sent riflemen to their doom
As Allies :
* Sandbags : Oh wow, tanks crush them silly, while they're cheap, I have never seen them of use, except in the trolling / joking game where they're actually used to draw something...
* Radar Jammer : Whats their use if your enemy actually TURN OFF THEIR RADAR ALL OF THE TIME?

As Soviets :
* Barbed wire : Oh lol, even rocket soldiers destroy them silly.
* MAD Tank : Demo truck done their job faster, and much more absolutely devastating if done right, you can argue about no killing infantry and less prone to (not so) accidental explosion inside your base, but still, they didn't insta-destroying some buildings (but demo truck definitely is), add the fact that they cost the same with demo truck, who will think the MAD is worth it after the infantry bug are fixed?

Aand... honourable mention :
* Hijacker : I think only a handful of players use them (Myself included of course), but the fact that they need APC to be able to effectively stealing harvesters, the fact that unlike GLA hijackers, they are both weak and cannot cloak themselves, the Soviet hijackers are useless as mean of ambush (unless you count moving them with APC and capturing mammoth tanks from behind is good, but the trick didn't work for the second time)